MONOGRAPHS  ON  EXPERIMENTAL  BIOLOGY 


EDITED  BY 

JACQUES  LOEB,  Rockefeller  Institute 

T.  H.  MORGAN,  Columbia  University 

W.  J.  V.  OSTERHOUT,  Harvard  University 


VOLUME  I 

FORCED  MOVEMENTS,  TROPISMS, 
AND  ANIMAL  CONDUCT 

BY 
JACQUES  LOEB,  M.D.,'  PH.D.,  Sc.D. 


ON  EXPERIMENTAL 


PUBLISHED 

VOLUME  I 

FORCED  MOVEMENTS,  TROPISMS,  AND  ANIMAL 
CONDUCT 

By  JACQUES  LOEB,  Rockefeller  Institute 
IN  PREPARATION 

THE  CHROMOSOME  THEORY  OF  HEREDITY 

By  T.  H.  MORGAN,  Columbia  University 

INBREEDING  AND  OUTBREEDING:  THEIR  GENETIC 
AND  SOCIOLOGICAL  SIGNIFICANCE 

By  E.  M.  EAST  and  D.  F.JONES,  Bussey  Institution,  Harvard  University 

PURE  LINE  INHERITANCE 

By  H.  S.  JENNINGS,  Johns  Hopkins  University 

THE  EXPERIMENTAL  MODIFICATION  OF  THE 
PROCESS  OF  INHERITANCE 

By  R.  PEARL,  Johns  Hopkins  University 

LOCALIZATION  OF  MORPHOGENETIC  SUBSTANCES 
IN  THE  EGG 

By  E.  G.  CONKLIN,  Princeton  University 

TISSUE  CULTURE 

By  R.  G.  HARRISON,  Yale  University 

PERMEABILITY  AND  ELECTRICAL  CONDUCTIVITY 
OF  LIVING  TISSUE 

By  W.  J.  V.  OSTERHOUT,  Harvard  University 

THE  EQUILIBRIUM  BETWEEN  ACIDS  AND  BASES  IN 
ORGANISM  AND  ENVIRONMENT 

By  L.  J.  HENDERSON,  Harvard  University 

CHEMICAL  BASIS  OF  GROWTH 

By  T.  B.  ROBERTSON,  University  of  Toronto 

THE  ELEMENTARY  NERVOUS  SYSTEM 

By  G.  H.  PARKER,  Harvard  University 

COORDINATION  IN  LOCOMOTION 

By  A.  R.  MOORE,  Rutgers  College 
OTHERS  WILL  FOLLOW 


MONOGRAPHS  ON  EXPERIMENTAL BIOLOGY 

FORCED  MOVEMENTS, 

TROPISMS,  AND  ANIMAL 

CONDUCT 


BY 

JACQUES  LOEB,  M.D.,  PH.D.,  Sc.D. 

MEMBER    OF   THE    ROCKEFELLER    INSTITUTE    FOR   MEDICAL    RESEARCH 


PHILADELPHIA  AND  LONDON 
J.  B.  LIPPINCOTT  COMPANY 


LIBRARY 


COPYRIGHT,    IQl8,   BY  J.   B.   LIPPINCOTT  COMPANY 


Eleetrotyped  and  Printed  by  J.  B.  Lippincott  Company 
The  Washington  Square  Press,  Philadelphia,  U.S.A. 


EDITORS'  ANNOUNCEMENT 

THE  rapidly  increasing  specialization  makes  it  im- 
possible for  one  author  to  cover  satisfactorily  the  whole 
field  of  modern  Biology.  This  situation,  which  exists  in 
all  the  sciences,  has  induced  English  authors  to  issue 
series  of  monographs  in  Biochemistry,  Physiology,  and 
Physics.  A  number  of  American  biologists  have  decided 
to  provide  the  same  opportunity  for  the  study  of 
Experimental  Biology. 

Biology,  which  not  long  ago  was  purely  descriptive 
and  speculative,  has  begun  to  adopt  the  methods  of  the 
exact  sciences,  recognizing  that  for  permanent  progress 
not  only  experiments  are  required  but  that  the  experi- 
ments should  be  of  a  quantitative  character.  It  will  be 
the  purpose  of  this  series  of  monographs  to  emphasize 
and  further  as  much  as  possible  this  development  of 
Biology. 

Experimental  Biology  and  General  Physiology  are  one 
and  the  same  science,  by  method  as  well  as  by  contents, 
since  both  aim  at  explaining  life  from  the  physico-chemical 
constitution  of  living  matter.  The  series  of  monographs 
on  Experimental  Biology  will  therefore  include  the  field 
of  traditional  General  Physiology. 

JACQUES  LOEB, 
T.  H.  MOKGABT, 

W.  J.  V.  OSTERHOUT. 


676386 


AUTHOR'S  PREFACE 

ANIMAL  conduct  is  known  to  many  through  the  roman- 
tic tales  of  popularizers,  through  the  descriptive  work 
of  biological  observers,  or  through  the  attempts  of  vital- 
ists  to  show  the  inadequacy  of  physical  laws  for  the 
explanation  of  life.  Since  none  of  these  contributions 
are  based  upon  quantitative  experiments,  they  have  led 
only  to  speculations,  which  are  generally  of  an  anthropo- 
morphic or  of  a  purely  verbalistic  character.  It  is  the 
aim  of  this  monograph  to  show  that  the  subject  of  animal 
conduct  can  be  treated  by  the  quantitative  methods  of 
the  physicist,  and  that  these  methods  lead  to  the  forced 
movement  or  tropism  theory  of  animal  conduct,  which 
was  proposed  by  the  writer  thirty  years  ago,  but  which 
has  only  recently  been  carried  to  some  degree  of  com- 
pletion. Many  of  the  statements,  especially  those  con- 
tained in  the  first  four  chapters  of  the  book,  are  familiar 
to  those  who  have  read  the  writer's  former  publications, 
but  so  much  progress  has  been  made  in  the  last  few  years 
that  a  new  and  full  presentation  of  the  subject  seemed 
desirable.  Chapters  V  to  XIII  and  Chapter  XVI  are 
partly  or  entirely  based  on  new  experiments. 

Only  that  part  of  the  literature  has  been  considered 
which  contributes  to  or  prepares  the  way  for  quantitative 
experiments. 


8  AUTHOR'S  PEEFACE 

The  writer  is  under  obligation  for  valuable  criticism 
to  his  wife,  to  Professor  T.  H.  Morgan,  and  to  Lieutenant 
Leonard  B.  Loeb,  who  were  kind  enough  to*  read  the 
manuscript.  J.  L. 

The  Rockefeller  Institute 

for  Medical  Research, 

New  York. 

March,  1918. 


CONTENTS 


/  I.  INTRODUCTION 13 

v  II.  THE  SYMMETRY  RELATIONS  OF  THE  ANIMAL  BODY  AS  THE  START- 
ING POINT  FOR  THE  THEORY  OF  ANIMAL  CONDUCT 19 

III.  FORCED  MOVEMENTS 24 

IV.  GALVANOTROPISM 32 

i/' V.  HELIOTROPISM.    THE  INFLUENCE  OF  ONE  SOURCE  OF  LIGHT...     47 

1.  General  Facts 47 

2.  Direct  Proof  of  the  Muscle  Tension  Theory  of  Heliotrop- 

ism  in  Motile  Animals 52 

3.  Heliotropism  of  Unicellular  Organisms 62 

4.  Heliotropism  of  Sessile  Animals 63 

VI.  AN  ARTIFICIAL  HELIOTROPIC  MACHINE 68 

VII.  ASYMMETRICAL  ANIMALS 70 

VIII.  Two  SOURCES  OF  LIGHT  OF  DIFFERENT  INTENSITY 75 

IX.  THE  VALIDITY  OF  THE  BUNSEN-ROSCOE  LAW  FOR  THE  HELIO- 
TROPIC REACTIONS  OF  ANIMALS  AND  PLANTS 83 

X.  THE  EFFECT  OF  RAPID  CHANGES  IN  INTENSITY  OF  LIGHT 95 

XI.  THE  RELATIVE  HELIOTROPIC  EFFICIENCY  OF  LIGHT  OF  DIFFER- 
ENT WAVE  LENGTHS 100 

XII.  CHANGE  IN  THE  SENSE  OF  HELIOTROPISM 112 

XIII.  GEOTROPISM 119 

XIV.  FORCED    MOVEMENTS    CAUSED   BY    MOVING   RETINA   IMAGES: 

RHEOTROPISM  :    ANEMOTROPISM 127 

XV.  STEREOTROPISM 134 

XVI.  CHEMOTROPISM 139 

XVII.  THERMOTROPISM 155 

XVIII.  INSTINCTS 156 

XIX.  MEMORY  IMAGES  AND  TROPISMS 164 

LITERATURE 173 

9 


ILLUSTRATIONS 

FIG.  PAGE 

1.  Forced  Position  of  Larva  of  the  Dragon  Fly  whose  Left  Cerebral 

Ganglion  is  Destroyed 30 

2.  Forced  Position  of  Shrimp  when  Galvanic  Current  Goes  from  Head 

to  Tail 34 

3.  Forced  Position  of  Shrimp  when  Positive  Current  Goes  from  Tail  to 

Head 35 

4.  Position  of  Legs  of  Shrimp  when  Current  Goes  Sideways  through  the 

Animal 37 

5-6.  Show  Same  Effects  of  Current  on  the  Common  Crawfish  as  Those 

on  Shrimp  in  Figs.  2  and  3 38 

7.  Diagram  Indicating  the  Orientation  of  the  Neurons  for  Flexor  and 

Extensor  Muscles  of  the  Right  and  Left  Legs 39 

8-9.  Diagram  Indicating  Orientation  of  Neurons  for  Flexor  and  Extensor 

Muscles  of  Third  and  Fifth  Pairs  of  Legs 40 

10.  Forced  Position  of  Amblystoma  Larva  Under  Influence  of  Galvanic 

Current  Going  Through  Animal  from  Head  to  Tail. . . . 41 

11.  Forced  Position  of  Amblystoma  Larva  When  Current  Goes  from  Tail 

to  Head 41 

12.  Tentacles  and  Manubrium  of  Jellyfish  Under  Influence  of  Galvanic 

Current ; 42 

13.  Strip  of  Jellyfish  Under  Influence  of  Galvanic  Current 42 

14.  Paramcedum  Under  Normal  Conditions 43 

15.  Current  Going  Through  Paramoecium 44 

16.  Showing  that  Positively  Heliotropic  Animals  Will  Move  from  Sun- 

light into  Shade  if  Illumination  of  Both  Eyes  Remains  the  Same.. . .  50 

17.  Position  of  Water  Scorpion  When  Right  Eye  is  Towards  the  Light .  .  53 

18.  Positions  of  Ranatra  When  Light  is  in  Front  and  Behind  Animal ....  54 

19.  Robber  Fly  Under  Normal  Conditions 55 

20.  Robber  Fly  with  Right  Eye  Blackened 56 

2 1 .  Position  of  Robber  Fly  when  Lower  Halves  of  Both  Eyes  are  Blackened  57 

22.  Position  of  Robber  Fly  when  Upper  Halves  of  Both  Eyes  are  Blackened  58 

11 


12  ILLUSTRATIONS 

FIG.  PAGE 

23.  Diagram  Showing  Position  of  the  Flagellum  as  Seen  in  a  Viscid 

Medium 62 

24.  Tube  Worms  in  Aquarium 63 

25.  Same  Animals  After  Position  of  Aquarium  was  Reversed 64 

26.  Polyps  of  Eudendrium  all  Growing  Towards  Source  of  Light 66 

27.  Fly  with  Right  Eye  Blackened 72 

28.  Diagram  of  Apparatus  Used  to  Produce  Differential  Bilateral  Light 

Stimulation 76 

29.  Diagram  to  Show  Method  of  Measuring  Trails 77 

30.  Diagram  for  Constructing  Direction  of  Motion  of  Larvae 80 

31.  Method  for  Proving  Validity  of  Bunson-Roscoe  Law 90 

32.  A  Glass  Plate 91 

33.  Difference  in  Gathering  Places  of  Animals 96 

34.  Method  of  Determining  the  Relative  Heliotropic  Efficiency  of  Two 

Different  Parts  of  the  Spectrum . .  .• 107 

35.  Geotropic  Curvature  of  Stems  of  Bryophyllum  calycinum 120 

36.  All  Stems  were  Originally  Straight  and  Suspended  Horizontally 121 

37.  When  the  Size  of  the  Leaf  is  Reduced  by  Cutting  Out  Pieces  from  the 

Middle 120 

38.  Effect  of  Cutting  off  Lateral  Parts  of  the  Leaves 121 

39.  Influence  of  Motion  of  the  Hand  on  a  Swarm  of  Sticklebacks  in  an 

Aquarium 132 

40.  The  Regenerating  Polyp  of  Tubularia  in  Contact  with  Glass  Wall  of 

Aquarium 137 

41.  Reactions  of  Chilononas  to  a  Drop  of  -fa  per  cent.  HC1 145 

42.  Method  of  Proving  the  Paramcecia  are  not  Positive  to  Acid  of  Low 

Concentration..                                                                                    .  146 


FORCED  MOVEMENTS, 

TROPISMS,  AND  ANIMAL 

CONDUCT 


CHAPTER  I 

INTRODUCTION 

THE  analysis  of  the  mechanism  of  voluntary  and 
instinctive  actions  of  animals  which  we  propose  to  under- 
take in  this  volume  is  based  on  the  assumption  that  all 
these  motions  are  determined  by  internal  or  external 
forces.  Our  task  is  facilitated  by  the  fact  that  the  over- 
whelming majority  of  organisms  have  a  bilaterally  sym- 
metrical structure,  i.e.,  their  body  is  like  our  own,  divided  / 
into  a  right  and  left  half. 

The  significance  of  this  symmetrical  structure  lies 
in  the  fact  that  the  morphological  plane  of  symmetry 
of  an  animal  is  also  its  plane  of  symmetry  in  physiological 
or  dynamical  respect,  inasmuch  as  under  normal  con- 
ditions the  tension  in  symmetrical  muscles  is  the  same, 
and  inasmuch  as  the  chemical  constitution  and  the  velocity 
of  chemical  reactions  are  the  same  for  symmetrical  ele- 
ments of  the  surface  of  the  body,  e.g.,  the  sense  organs. 

Normally  the  processes  inducing  locomotion  are  equal 
in  both  halves  of  the  central  nervous  system,  and  the  ten- 
sion of  the  symmetrical  muscles  being  equal,  the  animal 
moves  in  as  straight  a  line  as  the  imperfections  of  its 

13 


14  TEOPISMS 

locomotor  apparatus  permit.  If,  however,  the  velocity 
of  chemical  reactions  in  one  side  of  the  body,  e.g.,  in  one 
eye  of  an  insect,  is  increased,  the  physiological  symmetry 
of  both  sides  of  the  brain  and  as  a  consequence  the  equality 
,pf  tension  of  the  symmetrical  muscles  no  longer  exist.  The 
muscles  connected  with  the  more  strongly  illuminated  eye 
are  thrown  into  a  stronger  tension,a  and  if  now  impulses 
for  locomotion  originate  in  the~central  nervous  system, 
they  will  no  longer  produce  an  equal  response  in  the 
symmetrical  muscles,  but  a  stronger  one  in  the  muscles 
turning  the  head  and  body  of  the  animal  to  the  source 
of  light.  The  animal  will  thus  be  compelled  to  change  the 
direction  of  its  motion  and  to  turn  to  the  source  of  light. 
As  soon  as  the  plane  of  symmetry  goes  through  the  source 
of  light,  both  eyes  receive  again  equal  illumination,  the 
tension  (or  tonus)  of  symmetrical  muscles  becomes  equal 
again,  and  the  impulses  for  locomotion  will  now  produce 
equal  activity  in  the  symmetrical  muscles.  As  a  conse- 
quence, the  animal  will  move  in  a  straight  line  to  the 
source  of  light  until  some  other  asymmetrical  disturbance 
once  more  changes  the  direction  of  motion. 

What  has  been  stated  for  light  holds  true  also  if  light 
is  replaced  by  any  other  form  of  energy.  Motions  caused 
by  light  or  other  agencies  appear  to  the  layman  as  expres- 
sions of  will  and  purpose  on  the  part  of  the  animal, 
whereas  in  reality  the  animal  is  forced  to  go  where  carried 
by  its  legs.  For  the  conduct  of  animals  consists  of  forced 
movements. 

The  term  forced  movements  is  borrowed  from  brain 
physiology,  where  it  designates  the  fact  that  certain  ani- 
mals are  no  longer  able  to  move  in  a  straight  line  when 

a  We  are  speaking  of  positively  heliotropic  animals  exposed  to  only 
one  source  of  light. 


INTRODUCTION  15 

certain  parts  of  the  brain  are  injured,  but  are  compelled 
to  deviate  constantly  toward  one  side,  which  is  (accord- 
ing to  the  species  and  the  location  of  the  injury  in  the 
brain)  either  the  side  of  the  injury  or  the  opposite  side. 
The  explanation  of  these  forced  movements  is  that  on 
account  of  the  one-sided  injury  of  the  brain  the  tension 
of  the  symmetrical  muscles  is  no  longer  the  same.  As  a 
consequence,  the  impulses1  for  locomotion  which  are  equal 
for  symmetrical  muscles  will  cause  greater  contraction 

certain  muscles  of  one  side  of  the  body  than  in  the 
symmetrical  muscles  of  the  other  side,  and  the  animal  will 
no  longer  move  in  a  straight  line.!  The  only  difference 
between  the  forced  movements  induced  by  unequal  illu- 
mination of  the  two  eyes  and  by  injury  to  the  brain  is 
that  in  the  latter  case  the  forced  movements  may  last 
for  days  or  throughout  the  whole  life,  while  in  the  former 
case  they  last  only  as  long  as  the  illumination  on  the  two 
sides  of  the  body  is  unequal.  If  we  bring  about  a  per- 
manent difference  in  illumination  in  the  eyes,  e.g.,  by 
blackening  one  eye  in  certain  insects,  we  can  also  bring 
about  permanent  circus  motions.  This  shows  that  animal 
conduct  may  be  justly  designated  as  consisting  of  forced 
movements. 

The  idea  that  the  morphological  and  physiological 
symmetry  conditions  in  an  animal  are  the  key  to  the 
understanding  of  animal  conduct  demanded  that  the  same 
principle  should  explain  the  conduct  of  plants,  since  plants 
also  possess  a  symmetrical  structure.  The  writer  was 
able  to  show  that  sessile  animals  behave  toward  light 
exactly  as  do  sessile  plants ;  and  motile  animals  like  motile 
plants.  The  forced  orientations  of  plants  by  outside 
sources  of  energy  had  been  called  tropisms;  and  the 
theory  of  animal  conduct  based  on  the  symmetrical  struc- 


16  TROPISMS 

ture  of  their  body  was,   therefore,   designated  as   the 
tropism  theory  of  animal  conduct. 

We  started  with  symmetrical  animals  since  in  their 
case  the  analysis  jof  conduct  is  comparatively  simple ; 
the  results  obtained  in  the  study  of  these  symmetrical 
organisms  allow  us  also  to  understand  the  conduct  of 
asymmetrical  animals.  We  shall  see  that  the  principles 
underlying  their  conduct  are  the  same  as  in  the  case  of 
symmetrical  animals,  the  asymmetry  of  the  body  altering 
only  the  geometrical  character  of  the  path  in  which  the 
animal  is  compelled  to  move,  not,  however,  the  mechanism 
of  conduct.  While  a  perfectly  symmetrical  organism, 
possessed  of  positive  heliotropism,  moves  in  a  straight 
line  to  the  source  of  light,  the  path  deviates  from  the 
straight  line  in  the  case  of  an  asymmetrical  organism 
and  may  in  some  cases,  as,  e.g.,  in  Euglena,  be  a  spiral 
around  the  straight  line  as  an  axis.  Some  authors  have 
tried  to  use  asymmetrical  organisms  as  a  starting  point 
for  the  analysis  of  conduct,  but  since  it  is  impossible 
to  understand  the  conduct  of  the  asymmetrical  organisms 
unless  it  is  based  upon  that  of  the  symmetrical  animals, 
these  authors  have  been  led  to  anthropomorphic  specula- 
tions, such  as  "  selection  of  random  movements "  which, 
as  far  as  the  writer  can  see,  cannot  even  be  expressed  in 
the  language  of  the  physicist. 

Although  the  tropism  theory  of  animal  conduct  was  • 
offered  thirty  years  ago  285>  286> 287  its  acceptance  was 
delayed  by  various  circumstances.  In  the  first  place,  the 
majority  of  the  older  generation  of  biologists  did  not 
realize  that  not  only  the  methods  of  the  physicist  are 
needed  but  also  the  physicist's  general  viewpointpon- 
cerning  the  nature  of  scientific  e'$MiiaHoiLs*^irmany 
cases  the  problem  of  animal  conduct  is  treated  in  a  way 


INTRODUCTION  17 

which  corresponds  more  to  the  viewpoint  of  the^  intro- 
spective psychologist  than  to  that  of  the  physicist.  The 
attempts  to  explain  animal  conduct  in  terms  of  "  trial 
and  error "  or  of  vague  "physiological  states'7  may 
serve  as  examples.  None  of  these  attempts  have  led  or 
can  lead  to  any  exact  quantitative  experiments  in  the 
sense  of  the  physicist.  Other  biologists  have  still  more 
openly  adopted  an  anthropomorphic  method  of  explana- 
tion. If  pleasure  and  pain  or  curiosity  play  a  role  in 
human  conduct,  why  should  it  be  otherwise  in  animal 
conduct!  The  answer  to  this  objection  is  that  typical 
forced  movements  when  produced  in  human  beings,  as,  J 
e.g.,  in  Meniere  's  disease  or  when  a  galvanic  current  goes 
through  the  brain,  are  not  accompanied  by  sensations  of 
pleasure  or  pain,  and  there  is  no  reason  to  attribute  the 
circus  movements  of  an  animal,  after  lesion  of  the  brain 
or  when  one  eye  is  blackened,  to  curiosity  or  thrills  of 
delight.  An  equally  forcible  answer  lies  in  the  fact  that 
plants  show  the  same  tropisms  as  animals,  and  it  seems 
somewhat  arbitrary  to  assume  that  the  bending  of  a  plant 
to  the  window  or  the  motion  of  swarmspores  of  algae  to 
the  window  side  of  a  vessel  are  accompanied  or  deter- 
mined by  curiosity  or  by  sensations  of  joy  or  satisfaction. 
'And  finally,  since  we  know  nothing  of  the  sentiments  and 
sensations  of  lower  animals,  and  are  still  less  able  to  meas- 
ure them,  there  is  at  present  no  place  for  them  in  science. 
The  second  difficulty  was  created  by  the  fact  that  the 
Aristotelian  viewpoint  still  prevails  to  some  extent  in 
biology,  namely,  that  an  animal  moves  only  for  a  pur- 
pose, either  to  seek  food  or  to  seek  its  mate  or  to  under- 
take something  else  connected  with  the  preservation  of 

2 


18  TROPISMS 

the  individual  or  the  race.5  The  Aristotelians  had  ex- 
plained the  processes  in  the  inanimate  world  in  the  same 
teleological  way.  Science  began  when  Galileo  overthrew 
this  Aristotelian  mode  of  thought  and  introduced  the 
method  of  quantitative  experiments  which  leads  to  mathe- 
matical laws  free  from  the  metaphysical  conception  of 
purpose.  The  analysis  of  animal  conduct  only  becomes 
scientific  in  so  far  as  it  drops  the  question  of  purpose 
and  reduces  the  reactions  of  animals  to  quantitative  laws. 
This  has  been  attempted  by  the  tropism  theory  of  ani- 
mal conduct. 

i>  Tills  view  is  still  held,  especially  among  authors,  who  lean  more  or 
less  openly  to  vitalism,  e.g.,  v.  Uexkiill,  Jordan,  Franz,  Bauer,  Budden- 
brock,  and  others. 


CHAPTER  II 

THE  SYMMETEY  EELATIONS  OF  THE  ANIMAL 

BODY  AS  THE  STARTING  POINT  FOE  THE 

THEOEY  OF  ANIMAL  CONDUCT 

THE  starting  point  for  a  scientific  and  quantitative 
analysis  of  animal  conduct  is  the  symmetry  relations  of 
the  animal  body.  (Jhe  existence  of  these  symmetry  rela- 
tions reduces  the  analysis  to  a  comparatively  simple 
problem.  ; 

Organisms  show(two  forms  of  symmetry,  radial  sym- 
metry, for  which  jellyfish  and  the  stems  and  roots  of 
most  plants  Vffer  a  well-known  example,  and  lateral  sym- 
metry, such  as  exists  in  man  and  most  animalsyln  radial 
symmetry  the(peripheral  elements  are  distributed  equally 
about  an  axis  of  symmetry^  in  the  case  of  lateral  sym- 
metry the  ^peripheral  elements  are  distributed  equally 
to  the  right  and  left  of  the  plane  of  symmetry^  (or  the 
median  plane)  by  which  the  body  is  divided  into  a  right 
and  left  half.  The  importance  of  this  symmetrical  struc- 
ture lies  in  the  fact  that  the  morphological  plane  of  sym- 
metry is  also  the  dynamical  plane  of  symmetry  of  the 
organism.  ^Symmetrical  spots  of  the  surface  of  an  animal 
are  chemically  identical,  having  the  same  chemical  con- 
stitution and  also  the  same  quantity  of  reacting  masses. ) 
Thus  the  two  eyes  are  symmetrical  organs,  each  contain- 
ing the  same  photochemical  substances  in  equal  quantity. 
In  the  eye  itself  each  element  is  to  be  considered  as 
chemically  identical  with  the  symmetrical  point  in  the 
other  eye.  Hence,  if  the  two  eyes  are  illuminated  equally, 

19 


20  TEOPISMS 

the  photochemical  reaction  products  produced  in  the  same 
time  will  be  equal  in  both  eyes.  What  is  true  for  the  eyes 
is  true  for  all  symmetrical  elements  of  the  surface  of  an 
animal. 

The  median  plane  is  also  the  plane  of  symmetry  for  the 
muscles  and  the  muscular  activity  of  the  body.  Sym- 
metrical muscles  possess  under  equal  conditions  equal 
tension  and  symmetrical  muscles  are  antagonistic  to  each 
other  in  regard  to  moving  the  body  to  the  right  or  left. 

We  say  that  impulses  go  from  the  central  nervous 
system  to  the  muscles ;  and  from  the  surface  of  the  body 
to  the  central  nervous  system.  According  to  our  present 
knowledge  that  which  is  called  a/nervous_  impulse  seems 
to  consist  of  a  wave  of  chemical  reaction  traveling  along 
a  nerve  fiber.^  The  central  nervous  system  is  also  sym- 
metrical and,  moreover,  we  may  conceive  a  projection  of 
the  elements  of  the  surface  of  the  body  upon  the  ganglion 
cells  and  from  here  tt)  the  muscular  system  of  the  body. 
The  complications  in  this  system  of  projections  consti- 
tute the  difficulties  in  our  understanding  of  the  structure 
of  the  brain.  This  idea  of  a  projection  of  the  sense  organs 
or  the  surface  of  the  body  upon  the  brain  will  explain 
why  the  morphological  plane  of  symmetry  of  an  organism 
is  also  its  plane  of  symmetry  in  a  dynamical  sense.  When 
symmetrical  elements  of  the  eyes  are  struck  by  light  of 
the  same  wave  length  and  intensity,  the  velocity  of  photo- 
chemical reactions  will  be  the  same  in  both  eyes.  Sym- 
metrical spots  of  the  retina  are  connected  with  symmetri- 
cal elements  in  the  brain  and  these  in  turn  are  connected 
with  symmetrical  muscles.  As  a  consequence  of  the  equal 
photochemical  reactions  in  the  symmetrical  spots  of  the 
retina  equal  changes  are  produced  in  the  symmetrical 
brain  cells  with  which  the  are  connected,  and 


SYMMETRY  RELATIONS  21 

changes  in  tension  will  be  produced  in  the  symmetrical 
muscles  on  both  sides  of  the  body  with  which  the  active 
brain  elements  are  connected.a  On  account  of  the  sym- 
metrical character  of  all  the  changes  no  deviation  from 
the  original  direction  of  motion  will  occur.  If,  however,s~ 
one  eye  is  illuminated  more  than  the  other  eye,  the  influ- 
ence upon  the  tension  of  symmetrical  muscles  will  no 
longer  be  the  same  and  the  animal  will  be  forced  to  deviate 
from  the  original  direction  of  motion. 

We  have  thus  far  considered  only  the  relation  between 
right  and  left.    Aside  from  this  symmetry  relation  we  il 
have  polarity  relations,  between  apex  or  head  and  base  ' 
or  tail  end.     Just  as  we  found  that  the  morphological 
plane  of  symmetry  is  also  a  dynamical  plane  of  symmetry, 
we  find  that  with  the  morphological  polarity  head-tail  is 
connected  a  dynamic  polarity  of  motion,  namely,  forward 
and  backward.    This  will  become  clear  in  the  next  chapter 
on  forced  movements. 

Physiologists  have  long  been  in  the  habit  of  studying 
not  the  reactions  of  the  whole  organism  but  the  reactions 
of  isolated  segments  (the  so-called  reflexes).  While  it 
may  seem  justifiable  to  construct  the  reactions  of  the 

a  Physiologists  assume  that  stimulations  are  constantly  sent  from  the 
brain  to  the  muscles  and  that  this  maintains  their  tension,  v.  Uexkiill 
uses  the  term  that  "tonus  "  is  sent  out  to  the  muscle  and  that  the  brain 
is  a  reservoir  of  "  tonus  "  as  if  the  latter  were  a  liquid.  The  writer  wonders 
whether  it  might  not  be  wiser  to  substitute  for  such  metaphors  hypotheses 
in  terms  of  chemical  mass  action.  -'Constant  illumination  causes  a  sta- 
tionary process  in  photosensitive  elements  of  our  eye,  in  which  the  mass  of 
the  reaction  product  is  determined  by  the  Bunsen-Roscoe  law.  We  assume, 
moreover,  that  in  proportion  to  this  photochemical  mass  action  correspond- 
ing chemical  reactions  take  place  in  the  brain  elements  with  which  the  eyes 
are  connected;  and  that  as  a  consequence  corresponding  chemical  reactions 
take  place  in  the  muscles  by  which  the  tension  of  the  latter  is  determined. 
These  processes  in  the  muscles  may  possibly  consist  in  the  establishment  of 
a  definite  hydrogen  ion  concentration.  Such  hypotheses!  have  the  advantage 
over  the  "  stimulation "  hypothesis  that  they  can  be  tested  by  physico- 
chemical  measurements. 


22  TROPISMS 

organism  as  a  whole  from  the  individual  reflexes,  such 
an  attempt  is  in  reality  doomed  to  failure,  since  reactions 
produced  in  an  isolated  element  cannot  be  counted  upon 
to  occur  when  the  same  element  is  part  of  the  whole,  on 
account  of  the  mutual  inhibitions  which  the  different 
parts  of  the  organism  produce  upon  each  other  when  in 
organic  connection13;  and^t  is,  therefore,  impossible  to 
express  the  conduct  of  a  whole  animal  as  the  algebraic 
sum  of  the  reflexes  of  its  isolated  segments/) 

E.  P.  Lyon320  has  shown  that  if  the  tail  in  a  normal 
shark  be  bent  to  one  side  (without  changing  the  position 
of  the  head)  the  eyes  of  the  animal  move  as  promptly 
as  compass  needles  in  association  with  the  bent  tail 
around  the  same  axis  in  which  the  bending  occurs,  but  in 
an  opposite  sense.  On  the  convex  side  of  the  animal,  the 
white  of  the  eye  is  more  visible  in  front,  on  the  concave 
side  it  is  more  visible  behind ;  hence  the  former  has  moved 
backward,  the  latter  forward.  This  was  observed  not 
only  in  the  normal  fish  but  also  when  the  optic  and  audi- 
tory nerves  were  cut.  (The  central  nervous  system  acts 
as  one  unit.)  R.  Magnus  332  and  his  fellow-workers  have 
shown  that  an  alteration  in  the  position  of  the  head  of  a 
dog  inevitably  alters  the  tone  of  the  muscles  of  the  legs.c 
These  and  other  associations  and  mutual  inhibitions  make 
possible  that  simplification  which  allows  us  to  treat  the 

b  When  the  stem  of  a  plant  (e.g.,  Bryophyllum]  is  cut  into  as  many 
pieces  as  there  are  nodes,  each  node  will  under  the  proper  conditions  give 
rise  to  one  or  two  shoots.  If  we  leave  them  in  connection,  only  the  buds 
at  the  apical  end  will  grow  out,  the  rest  of  the  buds  remaining  dormant. 
The  whole  stem  acts  as  though  it  consisted  of  only  the  bud  situated  at  the 
apex. 

c  The  problem  of  coordination  will  form  the  subject  of  another  volume  in 
this  series  by  Professor  A.  R.  Moore,  and  for  this  reason  a  fuller  discussion 
of  work  on  coordination,  such  as  that  by  Sherrington  and  by  v.  Uexkull, 
may  be  reserved  for  Professor  Moore's  volume. 


SYMMETRY  RELATIONS  23 

organism  as  a  whole  as  a  mere  symmetry  machine,  a 
simplification  which  forms  the  foundation  of  the  tropism 
theory  of  animal  conduct. 

It  would,  therefore,  be  a  misconception  to  speak  of 
tropisms  as  of  reflexes,  since  tropisms  are  reactions  of 
the  organism  as  a  whole,  while  reflexes  are  reactions  of 
isolated  segments.  Reflexes  and  tropisms  agree,  how- 
ever, in  one  respect,  inasmuch  as  both  are  obviously  of 
a  purely  physico-chemical  character. 


CHAPTER  III 
FOKCED  MOVEMENTS 

(WHEN  we  destroy  or  injure  the  brain  on  one  side  we 
paralyze  or  weaken  the  muscles  connected  with  this  side.N 
CA.S  a  consequence  the  morphological  plane  of  symmetry 
ceases  to  be  the  dynamical  plane  of  symmetry)  and  the 
animal  has  a  tendency  to  move  in  circles  instead  of  in 
a  straight  line.  Suppose  a  fish  swimming  forward  by 
motions  of  its  tail  fin.  Normally  the  stroke  occurs  with 
equal  energy  to  the  right  and  to  the  left,  and  the  rudder 
action  of  the  tail  is  equal  in  both  directions,  but  after 
the  lesion  of  one  side  of  the  brain  the  stroke  and  the 
rudder  action  cease  to  be  the  same  in  both  directions,  it  is 
weakened  in  one  direction.  Hence  the  animal  instead  of 
swimming  in  a  straight  line  is  forced  to  deviate  contin- 
ually toward  one  side  from  the  straight  line  of  locomo- 
tion. We  speak  in  such  a  case  of  a  forced  motion. 

(When  we  destroy  the  ventral  portion  of  the  left  optic 
lobe  in  a  shark  (Scyllium,  canicula),  the  fish  no  longer 
swims  in  straight  lines  but  in  circles  to  the  right  (when  the 
right  optic  lobe  is  destroyed  it  swims  in  circles  to  the 
left).)  After  the  destruction  of  the  left  optic  lobe,  the 
muscles  on  the  left  side  of  the  tail  are  weakened  or  semi- 
paralyzed)  and  they  no  longer  produce  the  same  rudder 
action  as  the  muscles  on  the  right  side.  Hence  the  im- 
pulses (or  nerve  processes)  which  flow  in  equal  intensity 
to  the  muscles  on  both  sides  will  no  longer  produce  equally 
energetic  rudder  action  of  the  tail  to  the  right  and  to  the 
left,  but  the  muscles  turning  the  tail  to  the  right  will 
24 


FORCED  MOVEMENTS  25 

contract  more  powerfully  than  those  turning  it  in  the 
opposite  direction.  The  outcome  of  this  greater  rudder 
action  of  the  tail  when  moving  to  the  right  is  that  the 
fish  instead  of  swimming  in  a  straight  line  moves  in  a 
circle  to  the  right.290 

It  is  often  the  case  that  the  body  of  such  a  fish  even 
when  quiet  is  no  longer  straight  but  bent  in  a  circle,  the 
left  side  forming  the  convex  side ;  and  when  such  a  fish 
dies  and  rigor  mortis  sets  in  it  may  become  stiff  in  this 
position.    These  latter  observations  (prove  that  the  (circus 
movements  to  the  right  are  due  to  the  lowering  of  the 
tension  of  the  lateral  muscles  of  the  body  on  the  left  side 
of  the  fish.^  This  is  the  fundamental  fact  for  the  theory 
of  forced  movements — namely,  that  a  lesion  in  one  side      ^ 
of  the  brain  lessens  the  tension  of  the  muscles  on  one  side      /\ 
of  the  body ;  as  a  consequence  the  motions  of  the  animal 
become  difficult  or  impossible  in  one  direction  and  become—- 
easy in  the  opposite  direction. 

In  many  cases  the  motions  of  an  animal  depend  upon  . 
a  cooperative  activity  of  two  sets  of  appendages,  e.g.,*  Y~£ 
the  pectoral  fins  of  a  fish  or  the  legs  of  an  animal.  Such 
cooperative  or  associated  action  is  determined  by  the 
fact  that  the  same  nerve  center  supplies  antagonistic 
muscles  of  the  two  organs  (e.g.,  the  lateral  fins).  Thus 
the  same  nerve  impulse  causes  both  our  eyes  to  move 
simultaneously  to  the  right  or  to  the  left.  When  we  look 
to  the  right,  the  same  impulse  which  causes  the  contrac- 
tion of  the  rectus  externus  muscle  in  the  right  eye  causes 
a  contraction  of  the  rectus  internus  muscle  in  the  left 
eye.  These  two  muscles  then  are  associated. 

In  a  fish  like  the  shark  the  position  and  innervation 
of  the  eyes  differ  from  that  of  the  human  being.  In  the 
shark  the  eyes  are  not  in  front  but  on  the  side,  and  the 


26  TROPISMS 

muscles  which  lift  the  eye  on  one  side  are  associated 
with  those  which  lower  it  on  the  other  side  of  the  head. 
A  similar  association  exists  in  regard  to  the  pectoral  fins, 
the  muscles  which  lift  the  right  pectoral  fin  are  associated 
with  those  which  lower  the  left  one,  and  vice  versa.  When 
a  normal  shark  swims  the  two  pectoral  fins  work  equally 
and  the  fish  swims  without  rolling  over  to  the  right  or 
to  the  left. 

If  wre  destroy  in  a  shark  the  left  side  of  the  medulla 
oblongata  forced  changes  in  the  position  of  the  two  eyes 
and  the  two  pectoral  fins  will  follow.290  (There  are  in 
addition  correlated  changes  in  the  other  fins  and  the  rest 
of  the  body  which  we  will  omit  in  order  to  simplify  the 
presentation  of  the  subject.)  When  a  shark,  whose  left 
medulla  is  cut,  is  kept  in  a  horizontal  position,  its  left  eye 
looks  down  and  the  right  eye  looks  up.  This  change  of 
position  of  both  eyes  indicates  that  the  relative  tension 
between  the  muscles  of  the  eyes  has  changed.  In  the  left 
eye  the  tension  of  the  lowering  muscles  predominates  over 
that  of  their  antagonists,  in  the  right  eye  the  reverse  is 
the  case.  The  pectoral  fins  likewise  show  associated 
changes  of  position.  The  left  fin  is  raised  up  dorsally, 
the  right  is  bent  down  ventrally.  Since  we  know  that 
the  destruction  of  the  central  nervous  system  causes  a 
paralysis  of  muscles  and  not  the  reverse  we  must  con- 
clude that  the(9estruction  of  the  left  side  of  the  medulla 
in  a  shark  causes  a  weakening  or  partial  paralysis  of  the 
muscles  which  lower  the  left  fin  and  of  those  which  raise 
the  right  fin.  Hence  the  muscles  which  press  down  on  the 
water  will  press  harder  in  the  right  than  in  the  left  fin. 
When  such  an  animal  swims  rapidly,  it  will  come  under 
the  influence  of  a  couple  of  forces  which  must  produce 
a  rolling  movement  around  the  longitudinal  axis  of  its 


FORCED  MOVEMENTS  27 

body  toward  the  left.  These  rolling  motions  are  another 
well-known  type  of  forced  movements.  When  such  an 
animal  swims  slowly,  it  will  roll  more  than  a  normal  fish, 
but  it  will  not  roll  completely  around  its  longitudinal  axis. 
These  are  the  same  motions  which  were  observed  in  dogs 
by  Magendie  and  Flourens155  after  an  operation  in  the 
medulla  or  pons..  We  can  state,  ^heref ore,  that  the  rolling 
motions  are  caused  by  the  weakening  of  one  group  of 
(associated)  muscles  while  their  antagonists  are  not 
weakened.  ) 

It  is  of  interest  to  consider  the  nature  of  forced  move- 
ments after  injury  of  the  cerebral  hemispheres  in  a  dog. 
When  in  a  dog  one  of  the  cerebral  hemispheres  is  injured 
the  animal  immediately  after  the  operation  no  longer 
moves  in  a  perfectly  straight  line,  but  deviates  from  the 
straight  line  toward  that  side  where  the  brain  is  in- 
jured.178 When  the  left  hemisphere  is  injured  circus 
motions  toward  the  left  ensue.  If  one  offers  a  dog  which 
was  operated  in  the  left  cerebral  hemisphere  a  piece  of 
meat,  removing  it  as  fast  as  the  dog  approaches,  the 
dog  will  move  at  first  a  certain  distance  in  a  straight  line ; 
it  will  then  suddenly  turn  to  the  left  and  describe  a  com- 
plete circle,  moving  afterward  for  a  little  while  in  a 
straight  line  toward  the  meat  and  turning  again  through 
an  angle  of  360°  to  the  left,  and  so  on.284  The  explanation 
is  the  same  as  for  the  foregoing  cases.  The  lesion  of  the 
left  cerebral  hemisphere  caused  a  weakening  or  partial 
paralysis  of  the  muscles  which  turn  the  body  to  the  right. 
Hence  the  animal  will,  when  following  the  meat,  deviate 
to  the  left,  and  this  causes  a  displacement  of  the  retina 
image  in  the  same  direction  and  an  apparent  motion  of  the 
object  to  the  right.  We  shall  see  in  a  later  chapter  on 


28  TROPISMS 

the  orienting  effect  of  moving  retina  images  that  this 
deviation  of  the  retina  image  to  the  left  causes  a  forced 
motion  of  the  animal  to  the  right  which  compensates  its 
tendency  to  deviate  to  the  left  due  to  the  effect  of  the 
brain  lesion.  Hence  the  animal  approaches  the  meat  in 
an  approximately  straight  line.  But  it  does  so  with  diffi- 
culty and  sooner  or  later  tiring  of  this  effort  it  moves 
in  the  usual  automatic  way,  whereby  equal  impulses  reach 
the  muscles  on  both  sides.  This  results  in  a  complete 
circus  movement  to  the  left  on  account  of  the  weakening 
(caused  by  the  operation)  of  muscles  which  turn  the  body 
to  the  right.  The  retina  image  of  the  meat  again  induces 
a  straight  motion  and  the  whole  process  described  is 
repeated.  When  the  injury  to  the  brain  was  less  severe 
the  animal  may  follow  the  meat  for  long  distances  without 
turning  in  a  circle. 

When  such  a  dog  is  offered  simultaneously  two  pieces 
of  meat,  one  in  front  of  the  left,  the  other  in  front  of 
the  right  eye,  it  invariably  moves  toward  the  one  on  the 
left  side.  The  equal  flow  of  impulses  caused  by  the  sym- 
metrically located  pieces  of  meat  results  in  a  stronger 
contraction  in  the  muscles  on  the  left  than  on  the  right 
side  of  the  body,  since  as  a  consequence  of  the  lesion  the 
tension  of  the  former  muscles  is  greater  than  that  of  the 
latter.  When  two  pieces  of  meat  are  simultaneously 
offered  to  the  dog,  but  both  pieces  are  in  front  of  the  left 
eye,  the  dog  tries  to  get  the  piece  nearest  to  its  mouth, 
but  its  effort  carries  it  a  little  too  far  to  the  left  and  then 
it  takes  the  other  piece  of  meat  which  is  situated  farther 
to  the  left.284 

Some  time  after  the  operation  these  disturbances  may 
become  less  and  may  ultimately  disappear.  If  now  the 


FOECED  MOVEMENTS  29 

dog  is  operated  on  the  other,  e.g.,  the  right  hemisphere, 
circus  motions  to  the  right  appear. 

We  do  not  wish  to  exhaust  the  chapter  on  forced  move- 
ments but  may  perhaps  for  the  sake  of  completeness  point 
out  the  following  facts.  We  have  seen  that  if  one  cerebral 
hemisphere  is  injured  the  dog  shows  a  tendency  to  circus 
movements  to  the  operated  side.  When  both  hemispheres 
are  injured,  e.g.,  both  occipital  lobes  are  removed,  the  dog 
can  hardly  be  induced  to  move  forward  and  it  is  impos- 
sible to  cause  it  to  go  downstairs,  while  it  is  willing  to  go 
upstairs.  Its  front  legs  are  extended  and  its  head  is 
raised  high,  giving  the  impression  as  if  such  a  dog  had 
a  tendency  to  move  backward  rather  than  forward  or  that 
the  forward  movement  was  difficult.  If  the  two  anterior 
halves  of  the  cerebral  hemispheres  are  removed  the  re- 
verse happens.  The  animal  runs  incessantly  as  if  driven 
by  a  mad  impulse ;  its  head  is  bent  down  and  it  is  in  every 
respect  the  converse  of  the  animal  operated  in  the  occipi- 
tal lobes.  These  two  types  of  forced  movements  corre- 
spond to  the  morphological  polarity  tail-head.  This 
corresponds  to  the  idea  of  a  projection  of  the  surface 
elements  upon  the  brain  either  directly  or  by  crossing. 

These  three  types  of  forced  movements:  the  circus 
motions,  the  tendency  to  go  backward,  and  the  irresistible 
tendency  to  move  forward  will  appear  in  the  form  of  the 
tropistic  reactions  to  be  described  in  this  volume. 

Since  we  shall  deal  in  this  volume  chiefly  with  inverte- 
brates, it  may  be  of  importance  to  show  that  forced  move- 
ments can  also  be  produced  in  this  group  of  animals 
by  lesion  of  one  side  of  the  cerebral  ganglion,  and  that 
these  forced  movements  depend  also  upon  the  fact  that 
as  a  consequence  of  the  operation  the  tension  of  sym- 
metrical muscles  (which  is  equal  under  normal  condi- 


30 


TBOPISMS 


tions)  becomes  unequal.  Fig.  1,5,  gives  the  change  in  posi- 
tion of  the  body  and  of  the  legs  in  the  larva  of  a  dragon 
fly  (^Eschna)  after  the  left  half  of  the  cerebral  ganglion 
has  been  destroyed  (Matula  541).  Such  an  animal  moves 
in  a  circle  to  the  right.  The  longitudinal  muscles  con- 
necting the  segments  of  the  body  are  under  higher  tension 
on  the  right  side  of  the  body  than  on  the  left  and  the  body 


FIG.  1. — B,  forced  position  of  larva  of  the  dragon  fly  (dZschna)  whose  left  cerebral 
ganglion  is  destroyed.  The  body  is  convex  on  the  left  side,  due  to  a  relaxation  of  the  muscles 
connecting  the  segments  on  the  left  side.  The  position  of  the  legs  is  such  that  the  animal 
can  only  move  in  circles  to  the  right.  This  asymmetry  disappears  again  when  both  ganglia 
are  destroyed,  C.  A,  normal  animal.  (After  Matula.) 

is  bent  with  its  convex  side  to  the  left.  The  normally 
symmetrical  position  of  the  legs  (Fig.  1,  A)  is  now 
changed  in  such  a  way  (Fig.  1,  B)  that  the  animal  is  no 
longer  able  to  move  in  a  straight  line,  but  is  forced  to  move 
in  circles  to  its  right.  We  shall  see  later  that  similar 
changes  in  the  position  of  the  legs  are  produced  in  a  posi- 
tively heliotropic  insect  when  the  left  eye  is  blackened 
and  in  a  negatively  heliotropic  insect  when  the  right  eye 


FOECED  MOVEMENTS  31 

is  blackened.  Circus  motions  after  destruction  of  one 
cephalic  ganglion  in  an  insect  are  a  general  occurrence 
and  have  been  known  for  a  long  time. 

The  importance  of  these  forced  movements  caused  by 
lesion  of  the  brain  for  the  explanation  of  the  conduct  of 
normal  animals  lies  in  the  fact  that  the  latter  is  essen- 
tially a  series  of  forced  movements.  The  main  difference 
between  the  forced  movements  after  brain  lesion  and  the 
conduct  of  a  normal  animal  lies  in  the  fact  that  the 
former  are  more  or  less  permanent ;  while  in  the  normal 
animal  conduct  the  changes  in  the  relative  tone  of  sym- 
metrical muscles  leading  to  a  temporary  forced  movement 
are  caused  by  a  difference  in  the  velocity  of  chemical^ 
reactions  in  symmetrical  sense  organs  or  other  elements 
of  the  surface. 


\ 


CHAPTER  IV 

GALVANOTBOPISM 

WHEN  we  send  a  galvanic  current  lengthwise  through 
a  nerve,  at  the  region  near  the  anode  the  irritability  of 
the  nerve  is  diminished,  while  it  is  increased  near  the 
cathode.  The  condition  of  decreased  irritability  near 
the  anode  is  called  aneiectrotonus  and  the  increased  irrita- 
bility near  the  cathode  is  called  catelectrotonus.  When 
a  current  is  sent  through  an  animal,  those  nerve  elements 
which  lie  in  the  direction  of  the  current  will  have  an  ane- 
lectrotonic  and  a  catelectrotonic  region ;  while  the  nerves 
through  which  the  current  goes  at  or  nearly  at  right  angles 
are  not  affected.  Ganglia  or  nerve  tracts  in  the  anelectro- 
tonic  condition  will,  therefore,  act  as  if  they  were  tem- 
porarily injured,  and  hence  we  need  not  be  surprised  to 
find  that  the  galvanic  current  causes  forced  movements 
which  last  as  long  as  the  current  lasts,  and  which  cease 
with  the  current. 

Hermann  reported  in  1885 204  that  when  a  current 
is  sent,  through  a  trough  containing  tadpoles  of  a  frog, 
the  tadpoles  orient  themselves  in  the  direction  of  the 
current  curves  putting  their  heads  to  the  anode.a  Blasius 
and  Schweizer 523  found  soon  afterwards  that  a  large 
number  of  animals  when  put  into  a  trough  with  water 
through  which  a  galvanic  current  passes  have  a  tendency 
to  go  to  the  anode.  The  explanation  given  by  Hermann 
and  by  Blasius  and  Schweizer  is  not  correct.  They 

a  The  writer  has  never  been  able  to  repeat  this  observation. 
32 


GALVANOTROPISM  33 

assumed  that  the  current,  acting  upon  the  central  nervous 
system,  causes  sensations  of  pain  when  it  goes  in  the 
direction  from  tail  to  head  in  the  animal;  while  it  has  a 
soothing  or  hypnotizing  effect  when  it  goes  in  the  opposite 
direction,  namely  from  head  to  the  tail.  In  the  latter 
case  the  head  is  directed  toward  the  anode.  The  authors 
assume  that  the  animals  choose  the  position  with  least 
pain,  i.e.,  with  their  heads  to  the  anode.  This  assumption 
is  wrong,  since  we  know  that  when  a  galvanic  current  is 
sent  through  the  head  of  a  human  being  automatic 
motions  comparable  to  those  observed  in  animals  occur 
which  are  not  voluntary  and  which  are  unaccompanied 
by  any  pain  sensation.  Thus  when  a  galvanic  current  is 
sent  laterally  through  the  head,  the  person  falls  toward 
the  anode  side  but  has  no  feeling  of  pain.  Mach  noticed 
the  same  effect  of  falling  to  the  side  of  the  anode  when  a 
galvanic  current  was  sent  sidewise  through  fishes.830 
/These  galvanotropic  motions  are  in  reality  forced  move- 
ments/and this  has  been  proved  by  direct  observations. 
It  was  shown  by/Loeb/and  Maxwell  307/in  experiments 
on  crustaceans/and  by  Loeb  and  Garrey  306  on  salaman- 
aersihat  when  we  send  a  galvanic  current  through  ani- 
mals which  go  to  the  anode,  changes  in  the  position  of 
the  legs  occur  comparable  to  the  changes  in  the  position 
of  fins  and  eyes  mentioned  in  the  previous  chapter,  and 
that  these  changes  are  of  such  a  character  as  to.  make 
it  easy  for  the  animal  to  move  in  the  direction  of  the  anode 
and  difficult  if  not  impossible  to  move  in  the  opposite 
direction. 

In  all  these  experiments  it  is  of  importance  to  choose 
the  proper  density  of  the  current.    For  the  experiments  on 
the  shrimp  (Palamonetes)™1  the  animals  were  put  into  a 
3 


34  TBOPISMS 

square  trough,  two  opposite  sides  of  which  were  formed 
of  platinum  electrodes.  The  cross  section  of  the  fresh 
water  in  the  trough  was  1,400  mm.2  and  the  intensity 
of  the  current  about  1  milliarnpere  or  a  little  less.  We 
found  it  advisable  to  increase  the  intensity  very  gradually 
by  increasing  slowly  the  resistance  of  a  rheostat  in* a 
short  circuit  until  the  phenomenon  of  galvanotropism 
appeared  most  strikingly.  When  the  current  is  too  strong 
or  too  weak  the  phenomena  are  no  longer  clear.  The  com- 
mon shrimp  (Palcemonetes)  is  a  marine  crustacean  which 


FIG.  2. — Forced  position  of  shrimp  (Palcemonetes)  when  galvanic  current  goes  from 
head  to  tail.  Tension  of  extensor  muscles  of  tail  fin  prevails  over  that  of  flexors. 
Animal  can  swim  forward  (to  anode),  but  not  backward.  (After  Loeb  and  Maxwell.) 

lives  also  in  brackish  water  and  which 'can  stand  fresh 
water  long  enough  for  the  purpose  of  these  experiments. 
The  animal  can  swim  forward  as  well  as  backward;  in 
forward  swimming  the  extensor  muscles  of  its  tail  fin 
work  more  strongly  than  the  flexors  (Fig.  2) ;  in  swim- 
ming backward  the  flexors  work  energetically  (Fig.  3)  and 
thus  produce  a  powerful  stroke  forward,  while  the  ex- 
tensors contract  with  less  energy.  When  we  put  a  Palce- 
monetes  in  a  trough  through  which  a  current  goes  and  if 
we  put  the  animal  with  its  head  toward  the  anode  the  tail 
is  stretched  out  (Fig.  2).  This  means  that  the  tension  of 
the  extensor  muscles  prevails  over  that  of  the  flexors 
and  since  the  forward  swimming  is  due  to  the  stroke  of 


GALVANOTROPISM  35 

the  extensors,  and  since  it  is  antagonized  by  the  tension 
of  the  flexors,  the  animal  can  swim  forward  but  not 
backward,  or  only  with  difficulty;  if  we  put  the  animal 
with  its  head  toward  the  cathode  the  tail  is  bent  ventrally 
(Fig.  3),  which  means  that  the  tension  of  the  flexors  is 
stronger  than  that  of  the  extensors.  As  a  consequence 
the  animal  can  swim  backward  but  not  forward,  or  only 


FIG.  3. — Forced  position  of  shrimp  when  positive  current  goes  from  tail  to  head. 
Tension  of  flexors  of  tail  fin  prevails  over  that  of  extensors.  Animal  can  swim  backward 
(to  anode),  but  not  forward.  (After  Loeb  and  Maxwell.) 

with  difficulty.  In  both  cases  the  result  will  be  a  swim- 
ming of  the  animal  to  the  anode,  in  the  former  case  by 
swimming  forward  in  the  latter  by  swimming  backward. 
We  can  further  show  that  the  tension  of  the  muscles 
of  the  legs  of  Palcemonetes  is  always  altered  in  such  a 
sense  by  the  galvanic  current  that  motion  toward  the 
anode  is  facilitated,  while  that  toward  the  cathode  is 
rendered  difficult  or  impossible.  The  animal  uses  the 
third,  fourth,  and  fifth  pair  of  legs  for  its  locomotion 
(Fig.  2).  The  third  pair  pulls  in  the  forward  movement 


36  TKOPISMS 

and  the  fifth  pair  pushes.  The  fourth  pair  acts  like  the 
fifth  and  requires  no  special  discussion.  If  a  current  be 
sent  through  the  animal  longitudinally  from  head  to  tail 
and  the  intensity  be  increased  gradually,  a  change  soon 
takes  place  in  the  position  of  the  legs.  In  the  third  pair 
the  tension  of  the  flexors  predominates  (Fig.  2),  in  the 
fifth  the  tension  of  the  extensors.  The  animal  can  thus 
move  easily  by  pulling  of  the  third  and  by  pushing  of  the 
fifth  pair  of  legs,  that  is  to  say,  the  current  changes  the 
tension  of  the  muscles  in  such  a  way  that  the  forward 
motion  is  facilitated,  while  the  backward  motion  is  ren- 
dered difficult.  Hence  it  can  easily  go  toward  the  anode 
but  only  with  difficulty  toward  the  cathode.  If  a  current 
be  sent  through  the  animal  in  the  opposite  direction, 
namely  from  tail  to  head,  the  third  pair  of  legs  is  extended, 
the  fifth  pair  bent  (Fig.  3) ;  i.e.,  the  third  pair  can  push, 
the  fifth  pair  can  pull  backward.  The  animal  can  thus 
go  backward  with  ease  but  forward  only  with  difficulty. 
This  again  will  lead  to  a  gathering  of  such  animals  at 
the  anode,  this  time,  however,  by  walking  backward. 

The  phenomena  thus  far  described  recall  the  forced 
movements  mentioned  in  the  third  chapter,  where  certain 
injuries  of  the  brain  accelerate  forward  motion  while 
other  lesions  in  the  opposite  parts  of  the  brain  make 
forward  motion  difficult  if  not  impossible. 

Palcemonetes  can  also  walk  sidewise.  This  movement 
is  produced  by  the  pulling  of  the  legs  on  the  side  toward 
which  the  animal  is  moving  (contraction  of  the  flexors), 
while  the  legs  of  the  other  side  push  (contraction  of  ex- 
tensors). If  a  current  be  sent  transversely,  say  from  left 
to  right,  through  the  animal,  the  legs  of  the  left  side 
assume  the  flexor  position,  those  of  the  right  side  the 


aALVANOTROPISM 


37 


extensor  position  (Fig.  4).  The  transverse  current  thus 
makes  it  easy  for  the  animal  to  move  toward  the  left— 
the  anode — and  prevents  it  from  moving  toward  the  right 
— the  cathode.  If  a  galvanic  current  flows  transversely 


Fio.  4. — Position  of  legs  of  shrimp  when  current  goes  sidewise  through  the  animal, 
from  left  to  right.  In  the  legs  on  the  left  side  the  tension  of  the  flexors,  in  those  of  the  right 
side  the  tension  of  the  extensors  predominates.  The  animal  can  easily  go  to  the  left  (anode), 
but  not  to  the  right.  (After  Loeb  and  Maxwell.) 

through  the  animal,  it  creates  the  analogue  of  the  circus 
motions  produced  by  injury  of  one  side  of  the  brain. 
Figs.  5  and  6  show  that  the  current  produces  similar 
effects  in  the  crayfish  as  those  produced  in  the  shrimp 
(Figs.  2  and  3). 


38 


TEOPISMS 


It  is  not  difficult  to  suggest  by  aid  of  a  diagram  the 
arrangement  of  the  elements  in  the  central  nervous  system 
required  to  bring  about  the  phenomena  of  galvanotropism 
just  described  for  Palcemonetes.  We  take  it  for  granted 
that  the  regular  phenomena  of  anelectrotonus  and  cate- 
lectrotonus  of  motor  nerve  elements  suffice  for  the  ex- 
planation of  these  phenomena.  We  assume  that  if  the 
cell  body  of  a  neuron  is  in  the  state  of  catelectrotonus 


Fia.  5. 


FIGS.  5  and  6. — Show  the  same  effects  of  current  on  the  common  crayfish  as  those  on 
Palcemonetes  in  Figs.  2  and  3. 

its  activity  is  increased,  when  it. is  in  anelectrotonic  con- 
dition activity  is  diminished.  Neurons  having  the  same 
orientation  will  always  be  affected  in  the  same  sense  by 
the  current. 

Fig.  7  is  a  diagram  of  the  arrangement  of  neurons 
giving  rise  to  the  bending  of  the  legs  on  the  side  of  the 
anode  and  to  the  extension  of  the  legs  on  the  side  of  the 
cathode  when  the  current  goes  sidewise  through  the  ani- 
mal. This  diagram  assumes  that  the  nerves  innervating 
the  extensors  come  from  the  opposite  side  of  the  central 


GALVANOTROPISM 


39 


nervous  system,  while  those  innervating  the  flexors  are  on 
the  same  side.  This  diagram  corresponds  to  reality,  ac- 
cording to  the  histological  work  of  Allen.  When  the  cur- 
rent goes  from  right  to  left  through  the  crustacean  the  cell 
bodies  of  the  neurons  on  the  right  side  are  in  catelectro- 
tonus,  those  on  the  left  side  in  anelectrotonus.  The  for- 
mer are,  therefore,  in  a  state  of  increased  "irritability," 
the  latter  in  a  state  of  diminished  ' '  irritability. ' '  Hence 
the  flexors  of  the  right  leg  are  contracted  and  the  exten- 
sors relaxed,  while  the  flexors  of  the  left  leg  are  relaxed 
and  the  extensors  contracted. 


FIG.  7. — Diagram  indicating  the  orientation  of  the  neurons  for  flexor  and  extensor 
muscles  of  the  right  and  left  legs  to  explain  changes  of  position  of  legs  under  influence  of 
galvanic  current.  (After  Loeb  and  Maxwell.) 

Another  crustacean Gelasimus307  shows  the  same  effect 
of  the  current  when  it  goes  sidewise  through  its  body. 
When  the  thoracic  ganglion  from  which  the  nerves  of  the 
legs  originate  is  cut  longitudinally  in  the  middle,  all  the 
legs  assume  permanently  a  bent  position,  confirming  our 
assumption  that  the  extensor  nerves  cross  over  while 
the  flexors  originate  from  the  same  side  of  the  ganglion 
on  which  their  muscles  are.  It,  therefore,  looks  as  if  our 
diagram  were  the  expression  of  the  actual  condition. 

In  the  same  way  we  can  explain  the  results  of  a  gal- 
vanic current  when  it  goes  through  the  animal  length- 
wise. We  only  need  to  assume  that  the  cell  bodies  which 
send  their  fibers  to  the  flexors  of  the  third  pair  of  legs 


40  TROPISMS 

have  the  same  orientation  as  the  cell  bodies  which  send 
their  fibers  to  the  extensors  of  the  fifth  pair  of  legs 
(Fig.  8).  Hence  when  the  positive  current  goes  from 
head  to  tail  through  the  animal  (Fig.  8),  the  flexors  of 
the  third  pair  of  legs  and  the  extensors  of  the  fifth  pair 
must  be  thrown  into  greater  activity,  since  the  cell  bodies 
of  both  these  nerves  are  in  a  condition  of  catelectrotonus, 
i.e.,  increased  activity. 


Fio.  8. 


Fia.  9. 


FIGS.  8  and  9.— Diagram  indicating  orientation  of  neurons  for  flexor  and  extensor 
muscles  of  third  and  fifth  pairs  of  legs  to  explain  galvanotropic  reaction.  (After  Loeb 
and  Maxwell.) 

When  the  current  goes  from  tail  to  head  the  cell  bodies 
of  the  extensors  of  the  third  and  of  the  flexors  of  the  fifth 
pair  of  legs  are  in  catelectrotonus.  This  possibility  is 
expressed  in  the  diagram  Fig.  9. 

In  this  way  the  theory  of  the  galvanotropic  reaction 
of  those  animals  which  go  to  the  anode  seems  complete. 

What  has  been  demonstrated  for  Palcemonetes  holds 
not  only  for  many  crustaceans  but  for  vertebrates  also. 
Loeb  and  Garrey 306  have  shown  that  when  a  current 


GALVANOTEOPISM 


41 


is  sent  through  a  trough  containing  larvae  of  a  salamander 
(Amblystoma)  the  legs  and  head  of  the  larvae  assume 
definite  positions  depending  upon  the  direction  of  the 
current.  When  the  current  goes  from  head  to  tail  the 
legs  are  pushed  backward  and  the  head  is  bent  (Fig.  10) ; 


FIG.  10. — Forced  position  of  Amblystoma  larva  under  influence  of  galvanic  current 
going  through  animal  from  head  to  tail.  Head  down,  body  convex  on  dorsal  side.  Legs 
backward.  (After  Loeb  and  Carrey.) 

when  the  current  goes  from  tail  to  head  the  opposite 
position  is  observed  (Fig.  11).  The  analogy  with  the 
observations  on  Palcemonetes  is  obvious. 

Galvanotropic  reactions  are  found  throughout  the 
whole  animal  kingdom  and  the  following  observations 
made  by  Bancroft  on  a  jellyfish  (Poly orchis  penicillata) 


Fia.   11. — Forced  position  of  Amblystoma  larva  when  current  goes  from  tail  to  head.    Head 
raised,  legs  pushed  forward,  tail  raised.     (After  Loeb  and  Garrey.) 

are  of  especial  interest.16  Strips  containing  tentacles  and 
the  manubrium  were  cut  out  from  the  animal  and  put  into 
a  trough  through  which  a  current  flowed  of  25  to  0.200 
m.  a.  for  1  square  mm.  of  the  cross  section  of  the  liquid 
(dilute  sea  water)  in  the  trough. 


42 


TEOPISMS 


If  a  meridional  strip  passing  from  the  edge  on  one  side  through 
the  center  of  the  bell  to  the  other  edge  be  prepared  and  the  current 
passed  through  transversely,  tentacles  and  manubrium  turn  and  point 
toward  the  cathode  (Fig.  12).  A  reversal  of  the  current  initiates  a 
turning  of  these  organs  in  the  opposite  direction,  which  is  usually  com- 
pleted in  a  few  seconds.  This  can  be  repeated  many  times  and  the 
tentacles  continue  to  respond  after  hours  of  activity.  The  manubrium, 

however,  tires  sooner  and  fails  to  re- 
spond. If  the  strip  is  placed  with  its 
subumbrella  surface  upward  and  ex- 
tended in  a  straight  line  parallel  to  the 
current  lines,  the  making  of  the  current 
causes  the  tentacles  at  the  anode  end 
to  turn  through  an  angle  of  180°  and 
point  toward  the  cathode.  The  ten- 
tacles at  the  cathode  end  become  more 
crowded  together,  reminding  one  of  the 
tip  of  a  moistened  paint  brush,  and 
also  point  more  directly  toward  the 
cathode  (Fig.  13).  The  experiment 
may  be  varied  in  still  other  ways  by 

cutting  smaller  or  larger  pieces  from  the  edge  of  the  swimming  bell, 
but  the  response  is  always  the  same.  The  tentacles,  wherever  pos- 
sible, and  to  a  less  extent  the  manubrium,  bend  so  as  to  point 
toward  the  cathode.  The  response  depends  in  no  way  upon  the  con- 
nection of  these  organs  with  the  swimming  bell,  muscles,  or  nerve 
ring,  for  it  is  obtained  equally  well  with  isolated  tentacles  and 
pieces  of  tentacles.  Isolated  tentacles  when  placed  transversely  to 


FIG.  12. — Tentacles  T  and  manu- 
brium M  of  a  jellyfish  (Poly orchis) 
under  influence  of  galvanic  current 
are  turned  to  the  negative  pole. 
(After  Bancroft.) 


FIG.   13. — Strip  of  jellyfish  showing  that  under  the  influence  of  galvanic  current  tentacles 
on  both  ends  point  towards  cathode.      (After  Bancroft.) 

the  current  lines  curve  so  as  to  assume  a  more  or  less  complete 
U-shape,  with  their  concave  side  toward  the  cathode.  When  placed 
parallel  to  the  current,  the  tentacles  do  not  curve.19 

The  latter  observation  shows  the  fact  that  the  whole 
reaction  is  due  merely  to  an  increase  in  the  tension  of 
the  muscles  on  the  cathode  side  of  the  organ. 

Phenomena  of  galvanotropism  can  be  observed  also 
in  infusorians.  Thus  Verworn493  observed  that  when 


GALVANOTROPISM  43 

a  current  goes  through  a  trough  containing  Paramcecia 
the  animals  will  all  move  toward  the  cathode.  The  mech- 
anism of  the  reaction  was  discovered  by  Ludloff.817  The 
locomotion  of  Paramcecium  is  brought  about  by  cilia. 
As  a  rule  the  cilia  are  directed  backward  (Fig.  14),  and  in 
their  normal  movement  they  strike 
powerfully  backward  and  are  retracted 
with  less  energy  to  their  normal  posi- 
tion. Since  their  powerful  stroke  is  back- 
ward the  animal  is  pushed  forward  in  the 
water.  Ludloff  and  Bancroft  17> 18  show 
that  if  a  Paramcecium  is  struck  sidewise 
by  the  current,  the  position  of  the  cilia 
on  the  cathode  side  is  reversed  inasmuch 
as  they  are  now  turned  forward.  On  the 
anode  side  they  continue  to  be  directed 
backward  (Fig.  15,  a).  Instead  of 
striking  symmetrically  on  both  sides 
of  the  animal,  the  cilia  on  the  cathode  side  strike  for- 
ward powerfully  while  those  on  the  anode  side  strike 
powerfully  backward.  The  animal  is  thus  under  the 
influence  of  a  couple  of  forces  which  turn  its  oral  pole 
toward  the  cathode  side.  As  soon  as  it  is  in  this  condition 
the  symmetrical  cilia  are  struck  at  the  same  angle  by  the 
parallel  current  lines  and  they  must  assume  a  symmetri- 
cal position  which  is  as  in  Fig.  15,  b,  namely  the  cilia  are 
pointed  forward  toward  the  cathode  at  the  oral  end,  and 
backward  toward  the  anode  at  the  aboral  end.  As  long 
as  the  current  is  not  too  strong,  the  oral  region,  where 
the  cilia  are  pointing  forward,  is  rather  small  and  there- 
fore the  action  of  those  cilia  prevails  which  are  in  the 
majority  and  which  are  pointed  backward.  As  a  result 
the  organism  moves  slowly  forward  to  the  cathode. 


44 


TROPISMS 


A  similar  mechanism  of  galvanotropic  conduct  exists 
in  Volvox  a  spherical,  unicellular  organism  which  is  sur- 
rounded by  cilia  on  its'  whole  surface.  A  definite  pole 
of  the  organism  is  always  foremost  in  all  locomotions. 
This  organism  usually  swims  to  the  anode  when  in  a 
galvanic  field.  Bancroft  made  the  action  of  the  cilia  of 
Volvox  visible  with  the  aid  of  india  ink  and  was  able  to 
show  that  the  current  made  the  cilia  on  the  anode  side 
stop,  while  those  on  the  cathode  side  continue  to  beat.20 


FIG.  15. — a,  current  going  from  left  to  right  through  Paramascium,  the  position  of 
cilia  on  the  cathode  side  is  now  reversed,  their  free  ends  pointing  forward.  The  animal 
when  swimming  is  automatically  turned  with  its  oral  end  toward  the  cathode.  6,  current 
going  through  Paramcecium  from  aboral  to  oral  end.  Cilia  symmetrical  on  both  sides  but 
pointing  forward  at  oral  end  and  backward  at  aboral  end. 

Since  the  backward  stroke  is  always  the  effective  one  the 
organism  is  thus  carried  automatically  toward  the  anode. 
Terry478  found  that  Volvox  can  be  made  to  move 
toward  the  anode  as  well  as  toward  the  cathode.  It  moves 
to  the  anode  after  having  been  kept  in  the  dark  for  two 
or  three  days,  while  after  exposure  to  light  it  swims  to 
the  cathode.  Volvox  contains  chlorophyll  and  the  change 
in  the  sense  of  reaction  is  therefore  connected  with  the 
formation  of  a  product  of  chlorophyll  activity.  Bancroft 
found  that  when  Volvox  was  made  cathodic  by  exposure 
to  sunlight,  the  cilia  stop  on  the  cathode  side. 


GALVANOTROPISM  45 

While  the  locomotor  mechanism  of  unicellular  organ- 
isms, like  Paramcecia  and  Volvox,  is  as  simple  as  that 
of  higher  organisms,  the  locomotion  of  microorganisms 
possessing  only  one  flagellum,  like  Euglena,  is  more  com- 
plicated. It  was  generally  assumed  that  the  flagellum  acted 
like  a  single  oar  and  that  it  was  directed  forward,  but 
this  is  not  correct.  It  is  shaped  like  a  U  and  its  free 
end  is  directed  backward;  and  Bancroft  has  emphasized 
that  it  acts  by  the  formation  of  a  loop  which  moves  like 
a  wave  from  the  base  of  the  flagellum  to  its  free  tip.  The 
same  author  discovered  that  Euglena  are  galvanotropic 
when  raised  in  acid  media,  On  account  of  the  asymmetry 
of  their  locomotor  apparatus  they  are  compelled  to  swim 
in  a  spiral,  in  most  cases  to  the  cathode,  exceptionally  to 
the  anode.  Bancroft  showed  that  the  orientation  of  these  \ 
organisms  by  the  galvanic  current  is  identical  with  that 
by  light.21 

All  the  phenomena  of  galvanotropism  are,  therefore, 
reduced  to  changes  in  the  tension  of  associated  muscles 
or  contractile  elements,  as  a  consequence  of  which  the 
motion  of  the  organism  toward  one  pole  is  facilitated, 
while  the  motion  toward  the  opposite  pole  is  rendered 
difficult.  Galvanotropism  is,  therefore,  a  form  of  forced 
motions  produced  by  the  galvanic  current  instead  of  by  * 
injury  to  the  brain. 

There  remains  then  the  question  of  how  a  galvanic 
current  can  bring  about  those  changes  which  result  in 
the  anelectrotonic  and  catelectrotonic  condition  mentioned 
at  the  beginning.  Currents  can  pass  through  tissues  only] 
in  the  form  of  ions  whose  progress  is  blocked  by  mem- 
branes which  are  more  permeable  for  certain  salts  than 
for  others.  Those  salts  which  go  through  the  membrane 
carry  the  current  through  the  tissue  elements,  those 


46  TBOPISMS 

v 

which  do  not  go  through  will  increase  in  concentration 
at  the  surface  of  the  membrane.  It  is  the  latter,,  which 
cause  the  elect rotonic  effects ;  according  to  Lq^b  and 
Budgett 304  by  secondary  chemical  reactions  at  thp  boun- 
dary. Nernst  has  pointed  out  that  a  stationary;, condition 
must  arise  at  the  surface  of  the  membrane  due  to  the  fact 
that  the  increase  in  concentration  of  ions  by  the  electric 
current  gives  rise  to  a  current  of  diffusion  of  salt-  in  the 
opposite  direction  away  from  the  membrane.  ' '  The  aver- 
age change  of  concentration  at  the  membrane  depends, 
therefore,  upon  the  antagonistic  effects  of  the  current 
and  of  the  diffusion/'524  'This  must  be  kept  in  mind 
since  otherwise  the  effect  of  the  constant  current  should 
increase  constantly  with  its  duration,  which  is  not  the 
case,  on  account  of  the  establishment  of  a  condition  of 
equilibrium  between  the  increase  of  the  concentration  of 
ions  at  the  boundary  with  the  duration  of  the  current 
and  the  diminution  of  this  concentration  by  the  diffusion 
of  the  ions  in  the  opposite  direction  due  to  osmotic 
pressure. 


CHAPTER  V 

HELIOTROPISM 

THE  INFLUENCE  OF  ONE  SOURCE  OF  LIGHT 
1.  GENERAL  FACTS 

THE  fact  that  certain  animals  go  to  the  light  had,  of 
course,  been  known  for  hundreds  of  years,  but  this  was 
explained  in  an  anthropomorphic  way.  Thus  Lubbock, 
and  Graber,180  had  taken  it  lor  granted  that  certain 
animals  went  to  the  light  or  away  from  it  on  account 
of  fondness  for  either  light  or  darkness,  and  their  experi- 
ments were  calculated  to  demonstrate  this  fondness. 
Animals  were  distributed  in  a  box,  one-half  of  which 
was  covered  with  common  window  glass,  the  other  with 
an  opaque  body  or  with  colored  glass,  and  after  a  while 
the  number  of  animals  in  each  half  was  counted.  When 
the  majority  of  animals  were  found  in  the  dark  part  the 
animals  were  believed  to  have  a  preference  for  darkness; 
when  in  the  light  part  they  were  believed  to  be  fond 
of  light.  The  same  method  was  used  to  decide  whether 
animals  preferred  blue  to  red  or  vice  versa.  The  writer 
attacked  the  problem  from  the  physical  viewpoint,  assum- 
ing that  the  animals  are  "fonoT"  neither  of  light  nor  of 
"  darkness, "  but  that  they  are  oriented  by  the  light  in  a 
similar  way  as  plants  are ;  being  compelled  to  bend  or — as 
in  the  case  djf  motile  algae — move  automatically  either  to  a 
source  of  light-,  or  away  from  it.285' 287 

In  the  case. of  unequal  illumination  of  the  two  eyes  the 
tension  of  the.  symmetrical  muscles  in  an  animal  becomes 

47 


48  TEOPISMS 

unequal.  In  this  condition  the  equal  impulses  of  locomo- 
tion will  result  in  an  unequal  contraction  of  the  muscles 
on  both  sides  of  the  animal.  As  a  consequence  the  animal 
will  turn  automatically  until  its  plane  of  symmetry  is  in 
the  direction  of  the  rays  of  light.  As  soon  as  this  happens 
the  illumination  of  both  eyes  and  the  tension  of  sym- 
metrical muscles  become  equal  again  and  the  animal  will 
now  move  in  a  straight  line — either  to  or  from  the  source 
of  light.  What  appeared  to  the  older  authors  as  the 
expression  of  fondness  for  light  or  for  darkness  was 
according  to  the  writer's  theory  the  expression  of  an 
influence  of  light  upon  the  relative  tension  of  symmetrical 
muscles.  ^ 

Animals  which  are  compelled  to  turn  and  move  to  the 
source  of  light  the  writer  called  positively  heliotropic, 
those  which  are  compelled  to  turn  and  move  in  the  oppo- 
site direction  he  called  negatively  heliotropic.  The 
designation  heliotropism  (or  phototropism)  was  chosen 
to  indicate  that  these  reactions  of  animals  are  of  the 
same  nature  as  the  turning  of  plants  to  the  light;  and  the 
writer  was  indeed  able  to  show  that  sessile  animals  bend 
to  the  light  as  do  plants  which  are  raised  near  a  win- 
dow;288 while  motile  animals  move  to  (or  from)  a  source 
of  light  as  do  the  motile  swarmspores  of  algae  or  motile 
algae  themselves. 

We  will  first  discuss  positively  heliotropic  motile  ani- 
mals. The  positively  heliotropic  caterpillars  of  Porthesia 
chrysorrhcea 288  or  the  winged  plant  lice  of  Cineraria  288 
or  the  newly  hatched  larvae  of  the  barnacle  183  were  used 
by  the  writer  in  his  earliest  experiments  and  they  may 
serve  as  examples.  The  larvae  of  Porthesia  must  be  used 
after  hibernation  before  they  have  taken  food.  When 
about  50  or  100  of  such  larvae  are  put  into  a  test  tube  and 


HELIOTEOPISM  49 

the  latter  is  placed  at  right  angles  against  a  window,  all 
the  animals  begin  to  move  to  the  window  in  as  straight 
a  line  as  the  imperfections  of  their  locomotion  and  col- 
lisions permit.  As  soon  as  they  reach  the  window  side 
of  the  test  tube  they  remain  there  permanently,  unless 
the  test  tube  is  turned  around.  If  we  turn  the  test  tube 
around  an  angle  of  180°  the  animals  go  at  once  to  the 
window  again.  They  react  in  this  way  whether  the  source 
of  light  is  sunlight,  diffused  daylight,  or  lamp  light ;  and 
this  can  be  repeated  indefinitely.  The  animals  are  slaves 
of  the  light.  These  experiments  are  typical  for  posi- 
tively heliotropic  motile  animals. 

When  the  animals  have  reached  the  window  end  of 
the  test  tubes  they  remain  there,  since  the  light  prevents 
them  from  going  back.  But  in  staying  there  they  may 
assume  any  kind  of  orientation,  thus  proving  that  the  light 
orients  them  only  as  long  as  they  are  in  motion.  The  light 
affects  the  tension  of  the  muscles  and  we  shall  see  later 
that  when,  the  animals  are  not  moving,  the  change  in  the 
tension  of  the  muscles  manifests  itself  by  changes  in  the 
position  of  the  legs,  which  is  noticeable  in  organisms  with 
comparatively  large  appendages. 

That  these  animals  do  not  go  to  the  light  because 
they  prefer  light  to  darkness  but  because  the  light  orients 
them  is  proved  by  the  fact  that  they  will  go  from  light 
into  the  shade  if  by  so  doing  they  remain  oriented  with 
their  heads  toward  the  source  of  light.287  Let  direct 
sunlight  S  fall  upon  a  table  through  the  upper  half  of  a 
window  (W,  Fig.  16),  the  diffused  daylight  D  through 
the  lower  half.  A  test  tube  ac  is  placed  on  the  table  in  such 
a  way  that  its  long  axis  is  at  right  angles  with  the  plane 
of  the  window;  and  one-half  ab  is  in  the  direct  sunlight, 
the  other  half  in  the  shade.  If  at  the  beginning  of  the 
4 


50 


TBOPISM'S 


experiment  the  positively  heliotropic  animals  are  in  the 
direct  sunlight  at  a,  they  promptly  move  toward  the  win- 
dow, gathering  at  the  window  end  c  of  the  tube,  although 
by  so  doing  they  go  from  the  sunshine  into  the  shade. 
This  experiment  shows  also  that  it  is  not  the  intensity 


Fia.  16. — Showing  that  positively  heliotropic  animals  will  move  from  sunlight  into  shade 
if  in  so  doing  the  illumination  of  the  two  eyes  remains  the  same. 

gradient  of  light  in  the  dish  which  makes  positively  helio- 
tropic animals  move  to  the  light,  but  that  difference  in 
intensity  on  both  sides  of  the  animal  which  is  caused  by 
the  screening  effect  of  the  animal's  own  body.  The  same 
holds  true  for  chemotropism. 

Thus   far  we  have  discussed  positively  heliotropic 


HELIOTBOPISM  51 

animals  only.  In  the  case  of  unequal  illumination  of  the 
two  eyes  or  of  the  two  sides  of  the  body  of  a  negatively 
heliotropic  animal  the  tension  in  the  muscles  turning  the 
animal  to  the  source  of  light  is  diminished.  The  impulses^ 
for  locomotion  which  are  equal  for  the  muscles  of  both 
sides  of  the  body  will,  therefore,  result  in  turning  the 
head  of  the  animal  away  from  the  source  of  light.  As 
soon  as  the  plane  of  symmetry  of  the  animal  goes  again 
through  the  source  of  light,  the  symmetrical  photosen- 
sitive elements  of  the  head  receive  again  equal  illumina- 
tion, and  the  animal  will  now  continue  to  move  in  a 
straight  line  away  from  the  source  of  light.  The  fully 
grown  larvae  of  the  housefly  when  they  are  ready  to  pupate 
show  this  negative  heliotropism. 

Negatively  heliotropic  animals,  e.g.,  the  fully  grown 
larvae  of  the  blowfly,  can  be  made  to  move  from  weak  light 
to  strong  light,  e.g.,  from  the  shade  into  direct  sunlight, 
if  in  so  doing  the  illumination  on  the  two  sides  of  the 
body  remains  equal.287  This  was  shown  by  the  writer 
by  a'fa  arrangement  similar  in  principle  to  the  one  de- 
scribetl  above.  Thus  the  idea  that  the  intensity  gradient 
of  light  determines  the  direction  of  motion  was  disproved 
also  for  negatively  heliotropic  animals. 

Thus  far  we  have  shown  only  that  a  heliotropic  animal^ 
is  oriented  in  such  a  way  to  a  source  of  light  that  its  plane 
of  symmetry  goes  through  the  source  of  light.    This  does  * 
not  yet  explain  why  a  positively  heliotropic  animal  cannot 
go  away  from  the  source  of  light,  since  in  going  to  or 
going  away  from  the  source  of  light  both  sides  of  the 
animal  receive  equal  illumination.    The  fact  that  a  posi- 
tively heliotropic  animal  cannot  go  away  from  the  light 
finds  its  explanation  by  observations  of  Holmes  228  and 
Garrey,177  showing  that  when  light  falls  from  behind 


52  TEOPISMS 

and  above  on  a  positively  lieliotropic  animal  its  progres- 
sive motions  are  stopped,  and  in  some  cases  a  tendency 
to  turn  a  somersault  backward  may  even  arise.  The  case 
is  similar  to  that  of  galvanotropism  when  the  current  goes 
through  an  animal  lengthwise  (see  previous  chapter). 
We  must  conclude  from  the  observations  of  Holmes  and 
Garrey,  which  will  be  discussed  farther  on,  that  if  the 
mead  of  a  positively  heliotropic  animal  is  turned  to  a 
/source  of  light  its  forward  motions  are  facilitated  and 
the  backward  motions  rendered  difficult ;  while  in  the  case 
of  a  negatively  heliotropic  animal  it  is  just  the  reverse. 
'  If  the  animal  now  moves  to  the  right  or  to  the  left  the 
illumination  of  the  two  eyes  or  of  the  two  sides  of  the 
body  becomes  different  again,  causing  a  forced  movement, 
whereby  the  plane  of  symmetry  of  the  moving  animal  is 
caused  to  go  through  the  source  of  light  again ;  with  the 
head  toward  the  source  of  light  when  the  animal  is  posi- 
tively heliotropic  or  away  from  it  when  it  is  negatively 
heliotropic. 

2.  DIRECT  PROOF  OF  THE  MUSCLE   TENSION   THEORY  OF 
HELIOTROPISM  IN  MOTILE  ANIMALS 

The  fact  that  light  causes  forced  movements,  like 
those  described  in  the  case  of  brain  lesions  and  of  galvano- 
tropism, has  been  proved  by  many  observers,  and  espe- 
cially clearly  by  Holmes  and  Garrey.  Holmes  worked 
on  the  positively  heliotropic  water  scorpion  Ranatra.228 
When  this  animal  is  illuminated  from  the  right  side,  the 
legs  on  the  right  side  of  the  body  are  bent  and  those  on 
the  left  side  extended  (Fig.  17).  This  effect  is  identical 
with  the  one  observed  in  Palcemonetes,  when  a  galvanic 
current  goes  sidewise  through  the  animal.  Hence  Ranatra 


HELIOTBOPISM 


53 


can  easily  move  to  the  source  of  light  on  its  right  side  but 
with  difficulty  or  not  at  all  in  the  opposite  direction. 

When  the  light  is  placed  behind  the  animal,  the  body 
is  raised  up  in  front  and  the  head  held  high  in  the  air 
(Fig.  18).  The  opposite  attitude  is  assumed,  when  the 
light  is  placed  in  front,  the  body  being  lowered  and  the 
head  bent  down  (Fig.  18).  These  effects  resemble  the 


FIG.   17. — Position  of  the  water  scorpion  Ranatra  when  the  right  eye  is  toward  the  light. 

(After  Holmes.) 

galvanotropic  effects  observed  in  the  position  of  the  head 
of  Amblystoma  when  the  current  goes  forward  or  back- 
ward through  the  animal. 

These  latter  observations  of  Holmes  explain,  as 
already  mentioned,  why  a  positively  heliotropic  animal 
cannot  move  away  from  the  light  and  why  a  negatively 
heliotropic  animal  cannot  move  to  a  source  of  light.  The 
progressive  motions  of  the  negatively  heliotropic  animal 
will  be  stopped  when  the  light  strikes  it  in  front ;  while 


54 


TEOPISMS 


these  motions  of  the  positively  heliotropic  animal  will  be 
facilitated  when  the  light  is  in  front  and  will  be  rendered 
impossible  when  the  light  is  behind. 

The  writer  had  observed  long  ago  that  when  the  con- 
vexity of  one  eye  is  cut  off  in  the  housefly  it  will  no  longer 
go  in  a  straight  line  but  will  make  circus  movements,  the 
normal  eye  being  directed  toward  the  center  of  the 
circle.286  It  was  shown  by  Parker  that  blackening  of  one 


FIG.  18. — The  lower  figure  represents  the  position  of  Ranatra  when  the  light  is  behind 
the  body.  The  upper  figure  represents  the  position  assumed  when  the  light  is  in  front jof 
the  animal.  (After  Holmes.) 

eye  of  the  positively  heliotropic  butterfly  Vanessa  antiopa 
calls  forth  circus  movements,  with  the  unblackened  eye 
toward  the  center  of  the  circle.398  Holmes,  Kadi,447 
Axenfeld,  Garrey,  m  and  many  other  authors  have  since 
made  similar  observations.  In  the  positively  heliotropic 
Ranatra,  Holmes  described  the  effect  of  blackening  one 
eye  as  follows : 

If  one  eye  of  Ranatra  is  blackened  over  or  destroyed  the  insect 
in  most  cases  no  longer  walks  in  a  straight  line  but  performs  more  or 
less  decided  circus  movements  toward  the  normal  side.  Under  the 
stimulus  of  light  the  insect  assumes  a  peculiar  attitude;  the  body  leans 
over  toward  the  normal  side  and  the  head  is  tilted  over  in  the  same 
direction.228 


HELIOTROPISM 


55 


This  is  the  combination  of  circus  movements  with  roll- 
ing movements  familiar  to  those  who  have  experimented 
on  the  brain  of  fish,  where  a  destruction  of  one  side  of  the 
midbrain  calls  forth  rolling  motions  as  well  as  circus 
motions  toward  the  same  side.  Holmes 's  observations 


FIG.   19. — Robber  fly  under  normal  conditions  seen  from  above.     (After  Carrey.) 

were  extended  by  Garrey's  experiments  on  a  large  num- 
ber of  insects.  Garrey  found  that  the  robber  fly  (Procta- 
canthus)  (Fig.  19),  which  is  positively  heliotropic,  is  an 
unusually  good  object  for  the  demonstration  that  the 
heliotropic  reactions  of  animals  are  of  the  type  of  forced 
movements.  When  one  eye  of  this  fly  is  blackened  the 
legs  on  the  side  of  the  unblackened  eye  are  flexed  and  the 


56 


TBOPISMS 


legs  on  the  side  of  the  blackened  eye  are  more  extended 
than  normally  and  spread  farther  apart.3-  The  body  may 
tilt  as  far  toward  the  side  of  the  unblackened  eye  as  to 
press  the  legs  to  the  table  (Fig.  20) .  There  is  sometimes  a 


FIG.  20. — Robber  fly  with  right  eye  blackened,  seen  from  above  as  in  Fig.  19.  The 
body  tilts  over  to  the  left  side  so  that  only  the  right  eye  is  visible  from  above.  Position  of 
legs  changed  in  such  a  way  as  to  make  motion  toward  left  possible,  toward  right  impossible. 
(After  Garrey.) 

tendency  on  the  part  of  the  body  of  the  animal  to  become 
slightly  concave  toward  the  side  of  the  unblackened  eye. 

Garrey  found  also  that  the  same  changes  take  place 
when  one  eye  receives  a  stronger  illumination  than  the 

a  Figs.  19  to  22  and  27  were  drawn  from  photographs  kindly  given  to 
the  writer  for  this  purpose  by  Professor  Garrey.  The  draughtsman  was 
unfortunately  not  familiar  with  the  anatomy  of  insects,  which  accounts  for 
shortcomings  in  the  drawings,  which,  however,  have  no  bearing  on  the  prob- 
lem for  which  the  drawings  are  intended. 


HELIOTBOPISM  57 

other.  Bringing  one  eye  into  the  bright  beam  of  light 
directed  through  the  objective  of  the  optical  system  of  the 
string  galvanometer,  while  the  other  eye  is  illuminated 
only  by  the  subdued  light  of  the  optical  room,  promptly 
produced  the  same  changes  in  the  position  of  the  legs 
and  body  which  were  observed  when  one  eye  was  black- 
ened, the  more  weakly  illuminated  eye  acting  like  the 
blackened  eye  in  the  former  experiment.  When  the  illu- 


FIG.  21. — Position  of  robber  fly  when  the  lower  halves  of  both  eyes  are  blackened.    Head 
tilted  up.     (After  Carrey.) 

mination  on  one  side  of  such  animals  is  stronger  than  on 
the  other  the  legs  on  the  more  strongly  illuminated  side  of 
the  animal  are  bent,  those  on  the  opposite  side  are  ex- 
tended ;  and  the  head  has  a  tendency  to  bend  toward  the 
light.  When  an  impulse  to  move  originates  in  the  animal, 
it  can  turn  easily  to  the  light  but  with  difficulty  in  the 
opposite  direction.  As  soon  as  its  head  is  turned  to  the 
source  of  light  and  both  eyes  receive  the  same  illumination 
the  difference  in  tension  of  the  legs  on  the  two  sides  of 
the  body  disappears  and  now  the  animal  moves  or  is 
carried  in  a  straight  direction  toward  the  light.  By  these 
experiments  the  proof  of  the  writer's  muscle  tension 
theory  of  heliotropism  is  made  complete.177 


58  TEOPISMiS 

Garrey  observed  that  when  the  lower  halves  of  the 
eyes  of  the  robber  fly  are  blackened  the  position  of  the 
legs  of  the  two  sides  is  symmetrical,  but  the  anterior  and 
middle  pairs  of  legs  are  extended  forward  to  the  maximal 
extent,  producing  a  striking  posture  in  which  the  anterior 
end  of  the  robber  fly  is  pushed  up  and  back  from  the  sur- 
face of  the  table.  The  body  is  in  opisthotonus  with  the 
abdomen  concave  on  the  dorsal  side,  while  the  head  is 
tilted  far  up  and  back  (Fig.  21). 


FIG.  22. — Position  of  robber  fly  when  upper  halves  of  both  eyes  are  blackened.     Head  down, 
body  convex  above.     (After  Garrey.) 

When  walking  these  robber  flies  gave  the  impression  of  trying  to 
climb  up  into  the  air.  The  wings  are  frequently  somewhat  spread  and 
the  animal  may  push  itself  up  and  back  until  poised  vertically  on  the 
tips  of  the  wings  and  abdomen.  The  tendency  to  fly  is  very  pronounced 
in  this  condition  and  upon  the  slightest  disturbance  the  fly  soars  upward 
and  backward,  striking  the  top  of  a  confining  glass  dish  or  completing 
a  circle  by  "  looping  the  loop  "  backward.  If  it  falls  upon  its  back 
it  rights  itself  by  turning  a  backward  somersault.  Unequal  blackening 
of  the  lower  parts  of  the  two  eyes  results  in  a  combination  of  the  effects 
just  described,  with  those  described  for  blackening  one  eye,  for  the 
animal  also  performs  circus  motions. 

With  the  upper  halves  of  the  eyes  blackened  the  attitude  is  the 
reverse  of  that  described  in  the  preceding  section  (Fig.  22).  The  an- 
terior and  middle  pairs  of  legs  are  flexed.  The  anterior  and  posterior 


HELIOTROPISM  59 

ends  of  the  body  bend  ventrally  with  the  body  in  emprosthotonus.  The 
head  is  bent  far  down.  The  animal  may  actually  stand  on  its  head,  but 
the  abdomen  retains  its  ventral  curvature,  leaving  a  considerable  angle 
open  between  its  dorsuni  and  the  wings  which  normally  rest  on  it. 

In  both  walking  and  flying  it  continually  keeps  close  to  the  table, 
and  upon  encountering  an  obstacle  it  frequently  does  a  forward  somer- 
sault. If  it  gets  on  its  back  it  rights  itself  with  greatest  difficulty  as  its 
efforts  simply  result  in  bending  the  tail  and  head  ventrally  until  they 
may  form  a  complete  ring.  In  galvanotropism  the  same  general  picture 
is  presented  by  Palcemonetes  and  Amblystoma  when  the  anode  is  at  the 
head  end,  the  tonus  changes  involved  being  identical  in  the  two  conditions 
(Garrey1"). 

These  experiments  leave  no  doubt  that  the  primary  | 
effect  of  light  consists  in  changes  in  the  tension  of  muscles 
and  that  the  heliotropic  reactions  which  appeared  to  the 
older  observers  as  voluntary  acts  are  in  reality  forced 
movements. 

in  the  chapter  on  forced  movements  after  brain  lesion 
the  fact  was  mentioned  that  a  dog  which  had  shown  circus 
movements  to  the  left  after  lesion  of  the  left  cerebral 
hemisphere  shows  circus  motions  to  the  right  when  after- 
ward the  right  hemisphere  is  injured  symmetrically;  in- 
stead of  being  a  physiologically  symmetrical  animal  again 
after  the  second  operation.  The  explanation  is  that  the 
new  operation  is  more  effective  than  the  old  one  whose 
effect  has  partly  worn  off.  Garrey  has  made  an  obser- 
vation on  heliotropism  which  shows  some  analogy  with 
this  experiment  on  the  brain. 

He  found177  that  "robber  flies  with  one  eye  black- 
ened show  the  postural  conditions  in  the  most  pronounced 
way  in  the  early  morning  or  after  being  kept  for  some 
hours  in  the  dark.  Constant  exposure  to  the  light  pro- 
duces considerable  fatigue  of  the  eye  with  recovery  in 
the  dark.  These  facts  among  others  suggested  the  possi- 


60  TEOPISMS 

bility  of  producing  a  different  sensitiveness  of  the  two 
eyes  and  corresponding  differences  in  the  muscle  tonus 
with  asymmetry  of  position,  and  in  physiological  action 
of  the  muscles  of  the  two  sides  of  the  body  when  the 
\  two  eyes  were  equally  illuminated.  Such  an  experiment 
I  constitutes  a  crucial  test  of  the  tonus  theory  of  helio- 
1  tropism.  It  succeeded  beyond  our  greatest  expectations. 
Asphalt  black  was  applied  to  the  right  eye  of  several 
specimens  of  P  root  acanthus.  In  two  or  three  days  the 
paint  had  formed  a  brittle  shell.  During  this  time  the 
blackened  eye  had  become  'dark  adapted. '  When  such  a 
fly  is  exposed  to  light,  it  tilts  and  circles  to  the  left.  If 
now  the  brittle  shell  is  cracked  off  the  right  eye  by  care- 
fully pinching  with  fine  forceps,  the  exposure  of  this  very 
sensitive  eye  to  light  results  in  a  reversal  of  the  whole 
picture;  the  fly  circles  toward  the  side  from  which  the 
black  was  removed.  Although  the  illumination  of  the 
two  eyes  is  of  equal  intensity,  what  was  the  normal  eye 
now  becomes  relatively  a  darkened  eye  owing  to  its  lesser 
sensitiveness.  A  differential  effect  results,  probably  due 
to  a  difference  in  the  rate  of  photochemical  change  in  the 
two  eyes.  This  reversal  of  the  muscle  tonus  and  of  forced 
motions  may  persist  for  an  hour  or  two  or  even  longer, 
until  the  two  eyes  become,  as  they  ultimately  do,  of  equal 
sensitiveness  and  the  fly  behaves  like  a  normal  animal. 

"  These  experiments  are  not  only  incompatible  with 
any  l avoidance'  idea,  for  after  removal  of  the  black  there 
is  nothing  to  avoid,  but  they  are  also  incompatible  with  the 
conception  of  *  habit  formation,'  for  '  habit'  in  the  per- 
formance of  the  circling  movements  is  of  no  avail  when 
light  is  admitted  to  the  darkened  eye — the  animals  circle 
to  that  side  because  the  tonus  of  the  muscles  is  such  that 
they  are  forced  to  do  so.  / 


HELIOTROPISM  61 

"All  the  experiments  show  that  the  muscle  tone  is  de- 
pendent upon  the  intensity  of  the  light  and  that  the 
postures  assumed  depend  upon  the  relative  difference  in 
the  light  stimulus  to  the  eyes.  In  animals  with  one  eye 
completely  covered  the  radii  of  the  circles  in  which  they 
moved  were  shorter  the  more  intense  the  illumination  of 
the  normal  eye.  With  one  eye  partially  covered  the 
circles  were  larger  than  when  completely  covered,  and  in 
the  same  way  the  circles  were  larger  when  one  eye  was 
covered  by  a  film  of  collodion  or  of  brown  shellac,  which 
admits  some  light,  than  when  subsequently  covered  by 
opaque  asphalt  black.  When  one  eye  was  partially  cov- 
ered by  central  application  of  the  black  paint  the  tilting 
and  circling  to  the  opposite  side  were  abolished  or  re- 
versed by  brilliant  illumination  of  the  partially  blackened 
eye.  These  results  explain  why  a  positively  heliotropic 
animal  with  one  eye  blackened  approaches  a  light  by 
a  series  of  alternating  small  and  large  circles,  the  former 
being  executed  when  the  good  eye  is  illuminated  from  the 
source  of  light,  the  larger  when  it  is  in  the  shadow." 

We  have  thus  far  discussed  chiefly  positively  helio- 
tropic animals,  i.e.,  animals  which  are  compelled  to  move 
toward  the  source  of  light.  The  difference  between  these 
and  negatively  heliotropic  animals  is  that  the  legs  on  the 
illuminated  sida_j^L  a  negatively  heliotropic  animal  are 
extended,  while  those  on  the  opposite  side  are  in  flexed 
position.  This  has  been  directly  observed  by  Holmes, 
who  also  made  sure  of  the  fact  that  negatively  heliotropic 
animals,  when  one  eye  is  blackened,  turn  in  circles  with 
the  blackened  eye  toward  the  center  of  the  circle 228 ;  while 
positively  heliotropic  animals  turn  in  circles  with  the 
unblackened  eye  toward  the  center  of  the  circle. 


62 


TKOPISMS 


3.  HELIOTROPISM  OF  UNICELLULAR  ORGANISMS 

In  unicellular  organisms,  where  cilia  act  as  locomotor 
organs,  it  can  easily  be  shown  that  the  orientation  by 
light  is  of  the  nature  of  changes  in  the  position  of  cilia; 
this  is  for  instance  the  case  in  respect  to  V'cfyox. 
Holmes226  states  for  the  heliotropic  reactions  of  this 
organism,  that  they  are  due  to  differences  in  the  activity 
of  the  cilia  on  both  sides  of  the  organism  and  this  ex- 
planation agrees  with  the  actual  observations  of  Bancroft 
on  the  galvanotropic  reactions  of  Volvox. 

In  flagellates,  the  mechanism  of  locomotion  is  very 
complicated  and  does  not  consist  in  an  oar-like  action  of 
a  flagellum  as  was  formerly  assumed.  Bancroft  has  shown 
that  in  Euglena,  as  already  stated,  the  flagellum  inserted 
at  the  anterior  end  of  the  organism  is  bent  backward  in 
the  form  of  an  inverted  U,  and  that  locomotion  is  brought 

about  by  the  formation  of  a 
loop  which  travels  from  the 
base  of  the  flagellum  toward  the 
free  end  (Fig.  23).  The  path 
of  the  organism  which  results 
from  this  action  is  a  spiral  with 
continual  rotation  of  the  organ- 
ism around  its  longitudinal 
axis.  Bancroft  has  shown  that 
the  behavior  of  the  organism 
under  the  influence  of  light  is 
identical  with  that  in  a  constant 
galvanic  field.21  One-sided  illu- 
mination as  well  as  a  current  going  transversally  through 
such  an  organism  cause  changes  in  the  position  of  cilia 
comparable  with  those  observed  in  the  legs  of  crustaceans, 
insects,  and  vertebrates. 


V 


FIG.  23. — Diagram  showing  the 
position  of  the  flagellum  as  seen  in  a 
viscid  medium,  a,  when  Euglena  is 
swimming  forward  in  a  narrow  spiral; 
6,  when  swerving  sharply  toward  the 
dorsal  side;  c,  when  moving  backward. 
(After  Bancroft.) 


HELIOTROPISM 


63 


4.  HELIOTROPISM  OF  SESSILE  ANIMALS 

When  we  study  the  effects  of  light  on  sessile  animals 
we  find  that  they  behave  in  a  similar  manner  to  sessile 
plants.  When  illuminated  from  one  side  they  bend  their 
heads  to  the  source  of  light  until  their  axis  of  symmetry 
goes  through  the  source  of  light.  In  this  case  the  sym- 
metrical photosensitive  elements  receive  equal  illumina- 
tion and  the  symmetrical  muscles  are  under  equal  tension. 
Hence  the  animal  remains  in  this  orientation.  These 
sessile  animals  were  the  first  examples  by  which  the 


FIG.  24. — Tube  worms  in  aquarium,  all  bending  toward  light. 

muscle    tension    theory    of    animal    heliotropism    was 
proved.288 

Spirographis  spallanzani  (Fig.  24)  is  a  marine  annelid 
from  10  cm.  to  20  cm.  long,  which  lives  in  a  rather  rigid 
yet  flexible  tube.  The  latter  is  formed  by  a  secretion 
from  glands  at  the  surface  of  the  animal.  The  tube  is 
attached  by  the  animal  with  its  lower  end  to  some  solid 
body,  while  the  other  end  projects  into  the  water.  The 
worm  lives  in  the  tube  and  only  the  gills,  which  are 
arranged  in  a  spiral  at  the  head  end  of  the  worm,  project 
from  the  tube.  The  gills,  however,  are  quickly  retracted, 
and  the  worm  withdraws  into  the  tube  when  touched  or 
if  a  shadow  is  cast  upon  it. 


64 


TBOPISMS 


When  such  tubes  with  their  inhabitants  are  put  into 
an  aquarium  which  receives  light  from  one  side  only,  it 
requires,  as  a  rule,  a  day  or  more  until  the  foot  end  of 
the  tube  is  again  attached  to  the  bottom  of  the  aquarium. 
As  soon  as  this  occurs,  the  anterior  end  of  the  tube  is 
raised  by  the  worm  until  the  axis  of  symmetry  of  the  gills 
falls  into  the  direction  of  the  rays  of  light  (Fig.  24)  which 
enter  through  the  window  into  the  aquarium.288  When 
the  animal  has  once  reached  this  position  it  retains  it  as 


Fia.  25. — The  same  animals  after  the  position  of  the  aquarium  to  the  window  was  reversed. 

long  as  the  position  of  the  aquarium  and  the  direction  of 
the  rays  of  light  remain  the  same.  When,  however,  the 
aquarium  is  turned  180°,  so  that  the  light  falls  in  from 
the  opposite  direction,  the  animal  bends  its  tube  during 
the  next  twenty-four  or  forty-eight  hours  in  such  a  way 
that  the  axis  of  symmetry  of  its  circle  of  gills  is  again 
in  the  direction  of  the  rays  of  light  (Fig.  25).  When 
the.  light  strikes  the  aquarium  from  above,  the  animals 
assume  an  erect  position,  like  the  positively  heliotropic 
stems  of  plants  when  they  grow  in  the  open. 

In  these  phenomena  the  mechanical  properties  of  the 
tube  play  a  role.  When  the  animal  is  taken  out  of  the 
bent  tube,  the  latter  retains  its  form.  This  permanent 


HELIOTEOPISM  65 

change  of  form  of  the  tube  is  apparently  caused  through 
the  secretion  of  new  layers  on  the  inside  of  the  tube.  The 
youngest  layers  of  the  secretion  are  more  elastic  than  the 
old  layers,  and,  moreover,  have  at  first  a  powerful  tend- 
ency to  shorten.  If  such  a  secretion  occurs  on  one  side  of 
the  tube  only,  or  more  so  than  on  the  opposite  side,  the 
former  must  become  shorter  than  the  latter,  and  the  result 
must  be  a  curvature  of  the  tube,  that  side  becoming  con- 
cave where  the  new  secretion  has  occurred. 

On  this  assumption  the  process  of  heliotropic  curva- 
ture is  in  this  case  as  follows :  when  the  light  strikes'  the 
circle  of  gills  from  one  side  only,  in  these  elements  certain 
photochemical  reactions  occur  to  a  larger  extent,  than  on 
the  opposite  side.  This  results  in  corresponding  altera- 
tions of  the  sensory  nerve  endings,  the  sensory  nerves  and 
the  corresponding  motor  nerves,  and  their  muscles.  The 
sense  of  these  changes  is  such  as  to  throw  the  muscles 
connected  with  the  nerves  of  the  gills  on  the  light  side 
into  a  more  powerful  tonic  or  static  contraction  than  the 
muscles  on  the  opposite  side  of  the  body.  The  consequence 
is  a  bending  of  the  circle  of  tentacles,  or  the  head,  toward 
the  source  of  light,  which  will  continue  until  the  axis  of 
symmetry  of  the  circle  of  tentacles  falls  into  the  direction 
of  the  rays  of  light.  When  this  happens,  symmetrical 
tentacles  are  struck  at  the  same  angle  (or  in  other  words, 
with  equal  intensity)  by  the  rays  of  light,  and  therefore 
the  tone  of  the  antagonistic  muscles  is  the  same.  The 
result  is  that  the  circle  of  tentacles  becomes  fixed  in  this 
position.  The  bending  of  the  head  produces  an  increased 
pressure  and  friction  of  the  animal  against  that  side  of 
the  tube  which  is  directed  toward  the  light,  and  this  pres- 
sure and  friction  lead  to  an  increased  secretion  and  the 
formation  of  a  new  layer  inside  the  tube. 

5 


66 


TBOPISMS 


Heliotropic  curvature  of  sessile  animals  can  be  shown 
equally  well  in  a  hydroid,  Eudendrium.  It  is  necessary  to 
cut  off  the  old  polyps  at  once  when  the  animal  is  brought 
into  the  laboratory  and  to  put  the  stem  into  fresh,  clear, 


FIG.  26. — Polyps  of  Eudendrium,  all  growing  toward  source  of  light.  The  arrow  indi- 
cates the  direction  of  the  rays  of  light,  which  in  one  case  fall  in  from  above,  in  the  ]other 
from  the  left  side. 

sea  water.  In  a  day  or  two  new  polyps  are  formed  by 
regeneration  and  these  polyps  will  bend  toward  the  light 
until  their  axis  of  symmetry  is  in  the  direction  of  the  rays 
of  light  (Fig.  26).  The  region  at  the  base  of  the  polyps 
is  contractile  and  when  light  strikes  the  polyps  from  one 


HELIOTBOPISM  67 

side  only,  the  stem  on  the  side  of  the  light  contracts  more 
than  on  the  other  side,  and  this  results  in  a  bending  of  the 
stem,  whereby  the  polyp  is  put  into  the  direction  of  the 
rays  of  light.  As  soon  as  the  axis  of  the  polyp  is  in  the 
direction  of  the  rays  of  light  (provided  there  is  only  one 
source  of  light),  the  tension  of  the  contractile  elements  is 
the  same  all  around,  and  there  is  no  more  reason  for  the 
organism  to  change  its  orientation.  It,  therefore,  re- 
mains in  this  orientation  to  the  light. 

The  muscle  tension  theory  of  animal  heliotropism  is, 
therefore,  proved  for  all  classes  of  the  animal  kingdom, 
infusorians,  hydroids,  annelids,  crustaceans,  etc.  It  would 
be  wrong  to  state  that  the  theory  holds  only  for  insects. 


CHAPTER   VI 

AN  AETIFICIAL  HELIOTEOPIC  MACHINE 

THE  reader  will  have  perceived  that  in  the  preceding 
analysis  animals  are  treated  as  machines  whose  appar- 
ently volitional  or  instinctive  acts,  as  e.g.,  the  motion 
toward  the  light,  are  purely  physical  phenomena.  The 
best  proof  of  the  correctness  of  our  view  would  consist 
in  the  fact  that  machines  could  be  built  showing  the  same 
type  of  volition  or  instinct  as  an  animal  going  to  the  light. 
This  proof  has  been  furnished  by  the  well-known  inventor, 
Mr.  John  Hays  Hammond,  Jr.  The  following  is  a  descrip- 
tion of  the  machine  given  by  one  of  Mr.  Hammond's 
fellow-workers  who  cooperated  with  him  in  the  develop- 
ment of  the  machine,  Mr.  B.  F.  Miessner. 

This  "  Orientation  Mechanism  "  consists  of  a  rectangular  box,  about 
3  feet  long,  1%  feet  wide,  and  1  foot  high.  This  box  contains  all  the 
instruments  and  mechanism,  and  is  mounted  on  three  wheels,  two  of 
which  are  geared  to  a  driving  motor,  and  the  third,  on  the  rear  end,  is  so 
mounted  that  its  bearings  can  be  turned  by  solenoid  electro-magnets 
in  a  horizontal  plane.  Two  5-inch  condensing  lenses  on  the  forward  end 
appear  very  much  like  large  eyes. 

If  a  portable  electric  light,  such  as  a  hand  flashlight,  be  turned  on  in 
front  of  the  machine  it  will  immediately  begin  to  move  toward  the  light 
and,  moreover,  will  follow  that  light  all  around  the  room  in  many  complex 
manoeuvres  at  a  speed  of  about  3  feet  per  second.  The  smallest  circle 
in  which  it  will  turn  is  about  10  feet  diameter;  this  is  due  to  the  limiting 
motion  of  the  steering  wheel. 

Upon  shading  or  switching  off  the  light  the  "  dog  "  can  be  stopped 
immediately,  but  it  will  resume  its  course  behind  the  moving  light  so 
long  as  the  light  reaches  the  condensing  lenses  in  sufficient  intensity.  In- 
deed, it  is  more  faithful  in  this  respect  than  the  proverbial  ass  behind 
the  bucket  of  oats.  To  the  uninitiated  the  performance  of  the  pseudo 
dog  is  very  uncanny  indeed. 

The  explanation  is  very  similar  to  that  given  by  Jacques  Loeb,  of 
reasons  responsible  for  the  flight  of  moths  into  a  flame.     .     .     . 
68 


HELIOTEOPIC  MACHINE  69 

The  orientation  mechanism  here  mentioned  possesses  two  selenium 
cells  corresponding  to  the  two  eyes  of  the  moth,  which  when  influenced 
by  light  effect  the  control  of  sensitive  relays  instead  of  controlling 
nervous  apparatus,  as  is  done  in  the  moth.  The  two  relays  (500  to  1,000 
ohm  polarized  preferred)  controlled  by  the  selenium  cells  in  turn  control 
electro-magnetic  switches,  which  effect  the  following  operations:  When 
one  cell  or  both  are  illuminated  the  current  is  switched  on  to  the  driving 
motor;  when  one  cell  alone  is  illuminated  an  electro-magnet  is  energized 
and  effects  the  turning  of  the  rear  steering  wheel.  The  resultant  turning 
of  the  machine  will  be  such  as  to  bring  the  shaded  cell  into  the  light. 
As  soon  and  as  long  as  both  cells  are  equally  illuminated  in  sufficient 
intensity,  the  machine  moves  in  a  straight  line  toward  the  light  source. 
By  throwing  a  switch,  which  reverses  the  driving  motors,  the  machine 
can  be  made  to  back  away  from  the  light  irra  most  surprising  manner. 
When  the  intensity  of  the  illumination  is  so  decreased  by  the  increasing 
distance  from  the  light  source  that  the  resistance  of  the  cells  approach 
their  dark  resistances,  the  sensitive  relays  break  their  respective  circuits 
and  the  machine  stops. 

The  principle  of  this  orientation  mechanism  has  been  applied  to  the 
"  Hammond  Dirigible  Torpedo  "  for  demonstrating  what  is  known  as 
attraction  by  interference.  That  is,  if  the  enemy  tries  to  interfere  with 
the  guiding  station's  control  the  torpedo  will  be  attracted  to  him,  ete.a 

Nothing  seems  to  have  been  published  beyond  these 
meagre  details,  but  the  writer  understands  that  the  active 
machine  has  been  demonstrated  in  a  number  of  places  in 
this  country.  It  seems  to  the  writer  that  the  actual  con- 
struction of  a  heliotropic  machine  not  only  supports  the 
mechanistic  conception  of  the  volitional  and  instinctive 
actions  of  animals  but  also  the  writer's  theory  of  helio- 
tropism,  since  this  theory  served  as  the  basis  in  the  con- 
struction of  the  machine.  We  may  feel  safe  in  stating 
that  there  is  no  more  reason  to  ascribe  the  heliotropic  reac- 
tions of  lower  animals  to  any  form  of  sensation,  e.g., 
of  brightness  or  color  or  pleasure  or  curiosity,  than  it  is 
to  ascribe  the  heliotropic  reactions  of  Mr.  Hammond's 
machine  to  such  sensations. 

&  Electrical  Experimenter,  September,  1915,  202. 


CHAPTER  VII 

ASYMMETRICAL  ANIMALS 

IT  was  necessary  for  us  to  begin  our  analysis  with 
symmetrical  animals  since  as  the  result  of  this  analysis 
the  conduct  of  asymmetrical  organisms  offers  no  difficulty. 
The  result  of  the  asymmetry  consists  merely  in  a  change 
in  the  geometrical  character  of  the  path  in  which  an  animal 
is  compelled  to  move  to  or  from  the  source  of -energy. 
While  this  path  is  a  straight  line  in  a  symmetrical  and 
positively  heliotropic  organism  it  is  a  spiral  around  this 
straight  line  as  an  axis  in  an  asymmetrical  organism, 
like  Euglena.  Suppose  a  positively  heliotropic  animal  to 
have  slightly  asymmetrical  appendages  which  give  it  a 
tendency  to  deviate  to  the  left.  Let  us  suppose  that  the 
plane  of  symmetry  of  the  animal  goes  at  the  beginning  of 
the  experiment  through  the  source  of  light  and  that  the 
animal  is  swimming  toward  the  light.  After  a  few  strokes 
the  head  of  the  organism  will  have  deviated  slightly  to  the 
left  on  account  of  the  asymmetry  in  the  activity  of  the 
appendages.  As  soon  as  the  median  plane  of  the  animal 
deviates  to  the  left,  the  left  eye  is  less  illuminated  than 
the  right  one.  As  a  consequence,  a  difference  in  the  ten- 
sion of  the  muscles  on  the  two  sides  of  the  animal  will  be 
produced  which  will  compensate  the  natural  lack  of  sym- 
metry in  the  muscles  and  the  animal  will  cease  to  deviate 
further  to  the  left;  and  this  compensating  effect  of  the 
unequal  illumination  of  the  two  eyes  will  continue  until 
the  animal  is  actually  oriented  in  the  right  way  again,  i.e., 
70 


ASYMMETRICAL  ANIMALS  71 

until  its  plane  of  symmetry  goes  through  the  source  of 
light.  All  that  the  inherited  or  accidental  asymmetry 
does  is  to  cause  the  animal  to  move  in  a  path  which  is  not 
a  mathematically  straight  line;  but  this  deviation  will 
be  marked  only  in  a  case  of  very  pronounced  or  excessive 
asymmetry. 

We  have  already  described  the  behavior  of  a  dog  whose 
left  cerebral  hemisphere  has  been  injured  and  who  has  a 
tendency  to  deviate  to  the  left.  When  such  a  dog  is  shown 
a  piece  of  meat  it  moves  toward  it  in  a  fairly  straight  line, 
its  tendency  to  deviate  to  the  left  being  compensated  by 
the  orienting  effect  of  the  retina  image  of  the  piece  of 
meat.  If  the  dog  deviates  to  the  left,  the  piece  of  meat  is 
apparently  dislocated  to  the  right  of  the  dog  and  this 
dislocation  alters  the  tension  of  the  muscles  on  the  two 
sides  of  the  animal  in  such  a  way  as  to  make  it  turn  back 
to  the  right.  In  this  way  the  dog  reaches  the  piece  of 
meat  in  a  fairly  straight  line,  though  with  a  greater 
amount  of  labor,  since  the  tendency  to  deviate  to  the  left 
is  constantly  compensated  automatically  by  a  stronger 
contraction  of  the  muscles  turning  the  animal  to  the  right. 

The  writer  showed  many  years  ago  that  many  insects 
have  a  tendency  to  creep  upward,  and  that  this  is  due  to 
an  orienting  effect  of  gravity  upon  the  animal.  When  a 
perfectly  symmetrical  insect  is  put  on  a  vertical  stick 
it  walks  upward  in  a  straight  line.  What  will  happen  when 
such  an  animal  is  made  asymmetrical1?  Garrey  has  per- 
formed this  experiment  by  using  flies  in  which  one  eye 
was  blackened.  As  we  have  seen,  such  organisms  are 
rendered  asymmetrical  not  only  in  regard  to  the  eyes 
but  also  in  regard  to  their  apparatus  of  locomotion,  since 
in  one  side  of  the  body  the  tension  of  the  flexors,  in  the 


72 


TROPISMS 


legs  of  the  other  side  the  tension  of  the  extensors  pre- 
vails. As  a  consequence  the  fly  has  a  tendency  to  move 
in  circles  with  the  intact  eye  toward  the  center. 

Grarrey  has  shown  that  when  a  fly  with  one  eye  black- 
ened is  put  on  a  vertical  stick,  it  still  walks  upward,  but  in 

spirals  around  the  stick  (Fig. 
27),  instead  of  in  a  straight 
line.  The  asymmetry  of  loco- 
motion changes  only  the  geo- 
metrical nature  of  the  path  in 
which  the  animal  moves,  from 
a  straight  line  to  a  spiral,  but 
does  not  alter  the  forced  move- 
ment character  of  the  reaction. 
Bancroft  has  pointed  out 
that  when  in  a  positively  helio- 
tropic  amphipod  one  eye  is 
blackened  and  the  legs  of  the 
same  side  are  cut  off,  the  ani- 
mal's path  would  be  a  combina- 
tion of  a  circus  motion  induced 
by  the  blackening  of  the  eye 
and  of  a  rolling  motion  around 
its  longitudinal  axis.  Both 
effects  combined  would  result 
in  the  animal  swimming  in  a 
spiral  path,  and  if  the  animal  is 
positively  heliotropic  it  would 
swim  in  such  a  path  toward  the 
light.  This  is  the  path  which  aquatic,  asymmetrical  posi- 
tively heliotropic  organisms,  such  as  the  flagellate 
Euglena,  describe  in  their  motions  to  the  light. 


FIG.  27. — Fly  with  one  (right)  eye 
blackened  can  creep  only  in  a  spiral 
on  a  vertical  stick,  while  normally  it 
creeps  in  a  straight  line.  (After 
Garrey.) 


ASYMMETRICAL  ANIMALS  73 

But  this  locomotor  mechanism  (of  Euglena)  is  imperfect,  it  forces 
the  organism  to  move  in  a  spiral,  and  always  to  turn  toward  a  structurally 
determined  side.  There,  are  many  organisms  which  swim  in  spirals  and 
become  oriented  by  turning  toward  a  structurally  defined  side.  Jen- 
nings and  Mast  include  all  such  orientations  under  "trial  and  error" 
and  contrast  them  with  the  direct  orientation  of  such  animals  as  the 
amphipods  in  which  the  turning  may  be  either  toward  the  left  or  the 
right.  Let  us  now  consider  whether  the  orientation  of  Euglena  is  more 
like  the  selection  of  random  movements,  (which  we  would  all  agree  may 
justifiably  be  called  "  trial  and  error"),  or  whether  it  is  more  like  the 
orientation  of  the  terrestrial  amphipods  studied  by  Holmes. 

I  think  that  all  students  of  behavior  including  Jennings  and  Mast 
believe  that  in  the  case  of  these  amphipods  we  have  direct  heliotropic 
orientation.  If  the  right  eye  of  such  a  positively  heliotropic  amphipod 
be  covered  with  asphalt  varnish  it  will  execute  circus  movements  towards 
the  left.  The  usual  explanation  is  that  the  main  nervous  connection  is 
between  the  eye  on  one  side  and  the  legs  on  the  opposite  side  of  the  body. 
The  light  shining  on  the  uncovered  eye  brings  about  a  condition  of  in- 
creased muscular  tonus  in  the  legs  of  the  opposite  side,  which  is  not 
present  in  the  legs  connected  with  the  covered  eye.  Consequently  the 
right  legs  push  more  strongly  and  the  amphipod  turns  towards  the  left. 

Suppose  now  we  remove  some  or  all  of  the  left  legs  from  an  amphipod 
of  this  kind  so  that  it  will  always  turn  toward  the  left,  and  transfer  it 
to  water  in  which  it  must  be  supposed  to  swim  in  a  spiral  path.  We 
will  then  have  an  organism  which  would  become  oriented  in  essentially 
the  same  way  that  Euglena  does.  The  animal  would  always  swerve 
toward  the  left.  But,  when  the  spiral  course  brings  it  into  such  a  posi- 
tion that  the  light  shines  directly  on  the  left  eye,  the  muscular  tonus  of 
the  right  legs  would  be  increased  and  the  swerving  toward  the  light  would 
increase.  Thus  orientation  would  be  effected  in  just  the  same  way  that 
it  is  in  Euglena. 

While  these  hypothetical  changes  that  must  be  made  in  the  amphipod, 
to  make  it  react  like  Euglena,  are  considerable,  they  concern  only  the 
details.  The  fundamental  nature  of  the  photochemical  substances,  the 
nature  of  their  stimulation  and  the  character  of  their  connection  with 
the  locomotor  organs  have  none  of  them  been  modified.  All  that  has 
been  done  is  to  make  an  asymmetrical  organism  swimming  in  a  spiral 
out  of  a  bilateral  one.a  These  changes  are  much  less  fundamental  than 

a  Swimming  in  a  straight  line. 


74  TEOPISMS 

those  which  we  would  have  to  imagine  in  order  to  make  an  amphipod 
orient  to  light  by  the  selection  of  random  movements.  In  order  to  bring 
about  this  latter  change  the  whole  nature  of  the  photochemical  sub- 
stances and  their  relations  to  the  leg  muscles  would  have  to  be  modified. 
In  the  one  case  the  required  changes  are  all  of  a  mechanical  nature  and 
so  simple  that  the  experiment  might  possibly  succeed.  In  the  other  case 
the  required  changes  are  largely  chemical,  and  so  complex  that  we  have 
no  data  for  even  imagining  what  ought  to  be  done  in  order  to  bring 
them  about  (Bancroft  21 ). 

The  asymmetry  of  organisms  only  modifies  the  geo- 
metrical character  of  the  path  but  not  the  mechanism  of 
the  reaction. 


CHAPTER  VIII 

TWO  SOUKCES  OF  LIGHT  OF  DIFFERENT 
INTENSITY 

THE  writer  observed  that  if  heliotropic  animals  are 
exposed  to  two  equidistant  lights  of  equal  intensity  they  I 
move  in  a  line  perpendicular  to  the  line  connecting  the' 
two  lights.287' 294  This  has  been  confirmed  by  numerous 
observers,  e.g.,  Bohn  on  Littorina,  by  Parker  and  his 
pupils,  especially  by  Bradley  M.  Patten  on  the  larvae  of  the 
blowfly,  by  Loeb  and  Northrop  on  the  motions  of  the  larvae 
of  Balanus,  and  by  others.  The  question  arises :  In  which 
line  will  an  animal  move  when  the  intensity  of  the  two 
lights  differs  ?  .  When  the  animal  is  positively  heliotropic 
it  should  cease  to  move  in  a  line  at  right  angles  to  the 
line  connecting  the  two  lights  but  should  move  in  a  line 
which  deviates  toward  the  stronger  of  the  two  lights ;  if 
the  animal  is  negatively  heliotropic  it  should  deviate 
toward  the  weaker  of  the  two  lights.  When  the  two  eyes 
are  illuminated  by  two  lights  of  different  intensity,  the 
illumination  in  both  eyes  can  become  approximately  equal 
only  when  the  eye  struck  by  the  weaker  light  is  exposed 
at  a  larger  angle  than  the  eye  struck  by  the  stronger 
light.  Under  such  conditions,  the  animal  should  be  com- 
pelled to  move  in  a  straight  line  which,  however,  is  no 
longer  at  right  angles  to  the  line  connecting  the  two  lights, 
but  which  deviates  to  an  extent  determined  by  the  differ- 
ence in  the  intensity  of  the  two  lights.  The  case  was 

75 


76 


TBOPISMS 


worked  out  quantitatively  by  Bradley  M.  Patten  on  a 
negatively  heliotropic  animal,  the  full  grown  larva  of 
the  blowfly.412,413  The  source  of  light  was  at  G  (Fig.  28) 
(one  or  more  Nernst  lamps  of  measured  candle  power), 
a  portion  of  light  from  these  lamps  passed  through  the 
screens  d  and  d'  to  the  mirrors  M  and  M',  set  at  a  definite 


FIG.  28. — Diagram  of  apparatus  used  to  produce  differential  bilateral  light  stimulation. 
O,  five  220-volt  Nernst  glowers;  M  and  M' ,  mirrors;  /  and  /',  central  point  of  mirrors;  O, 
center  of  observation  stage;  dotted  lines,  central  ray  of  beam  of  light  from  the  glowers 
reflected  to  O  by  the  mirrors;  d  and  d',  screens  with  rectangular  openings;  s  and  «',  light 
shields;  a  and  6,  2  c.p.  orienting  light  with  screens.  (After  Patten.) 

angle  so  that  the  rays  were  reflected  to  the  observation 
point  0.  The  two  beams  of  light  reaching  0  were  of  the 
same  intensity.  With  the  means  of  one  of  the  lights  at 
a  or  b  the  animal  was  first  caused  to  move  across  the 
field  0  at  right  angles  to  the  rays  reflected  from  the  mir- 
rors M  and  M'.  The  animals  first  started  in  this  direction, 
then  came  suddenly  under  the  influence  of  the  light  re- 


TWO  SOURCES  OF  LIGHT 


77 


fleeted  by  M  and  M'.  In  order  to  make  the  ratio  of  inten- 
sities of  light  from  M  and  M'  different,  the  observation 
stage  0  was  put  at  un- 
equal distance  from  M 
and  M'.  The  larvae  were 
made  to  record  their 
trails  while  moving 
under  the  influence  of 
two  lights  and  the  devi- 
ation of  this  trail  from 
the  perpendicular  upon 
the  line  connecting  the 
two  sources  of  light  M 
and  M'  was  measured 
with  a  goniometer  (Fig. 
29).  The  result  of  the 
measurements  of  2,500 
trails,  showing  the  pro- 
gressive increase  in  an- 
gular deviation  of  the 
larvae  (from  the  per- 
pendicular upon  the 
line  connecting  the  two 
lights)  with  increasing 
differences  between  the 
lights,  are  given  in 
Table  I.  Since  the  devi- 
ation or  angular  deflec- 
tion was  toward  the 
weaker  of  the  two  lights  (the  animal  being  negatively 
heliotropic)  the  deviation  is  marked  negative. 


Fio.  29. — Diagram  to  show  the  method  of 
measuring  trails.  The  lines  xy  and  x'y'  are 
drawn  through  the  trails  at  the  points  reached — 
marked  by  the  arrows — when  the  side  lights  were 
turned  on.  The  angle  of  deflection  from  this  line 
is  measured  by  a  protractor,  P.  The  small  figures 
near  the  arrows  indicate  the  number  of  wig-wag 
movements  made  when  the  side  lights  were 
turned  on;  1st  and  2nd  refer  to  the  sequence  in 
which  the  trails  were  run.  (After  Patten.) 


78 


TBOPISMS 

TABLE  I. 


Percentage  difference  in  the 
intensity  of  the  two  lights 


Average  angular  deflection  of  the 
two  paths  of  the  larvae  toward 
the  weaker  light 


Per  cent. 
0 

8% 

16% 
25 
331/3 
50 


831/3 
100 


Degrees 

-  0.09 

-  2.77 

-  5.75 

-  8.86 
-11.92 
-20.28 
-30.90 
-46.81 
-77.56 


Patten  also  investigated  the  question  whether  the  same 
difference  of  percentage  between  two  lights  would  give 
the  same  deviation,  regardless  of  the  absolute  intensities 
of  the  lights  used  (Weber's  law).  The  absolute  intensity 
was  varied  by  using  in  turn  from  one  to  five  glowers.  The 
relative  intensity  between  the  two  lights  varied  in  succes- 
sion by  0,  8  1/3,  16  2/3,  25,  33  1/3,  and  50  per  cent.  Yet 
the  angular  deflections  were  within  the  limits  of  error 
identical  for  each  relative  difference  of  intensity  of  the 
two  lights,  no  matter  whether  1,  2,  3,  4,  or  5  glowers  were 
used.  Table  II  gives  the  results. 

TABLE  II 

A  TABLE  BASED  ON  THE  MEASUREMENTS   OF  2,700  TRAILS  SHOWING  THE 
ANGULAR  DEFLECTIONS  AT  FIVE   DIFFERENT  ABSOLUTE    INTENSITIES 


Number 


Difference  of  intensity  between  the  two  lights 


of  glowers 

0 
per  cent. 

8^ 
per  cent. 

16^ 
per  cent. 

25 

per  cent. 

33^ 
per  cent. 

50 
per  cent. 

1 

2 
3 

4 
5 

-0.55 
-0.10 
+0.45 
-0.025 
-0.225 

Defle 
-2.32 
-3.05 
-2.60 
-2.98 
-2.92 

ction  in  de 
-5.27 
-6.12 
—5.65 
-6.60 
-5.125 

grees 
-9.04 
-8.55 
-8.73 
-9.66 
-8.30 

-11.86 
-11.92 
-13.15 
-11.76 
-10.92 

-19.46 
-22.28 
-20.52 
-19.88 
-19.28 

Average.  .  . 

-0.09 

-2.77 

-5.75 

-8.86 

—11.92 

-20.28 

TWO  SOURCES  OF  LIGHT  79 

On  the  writer's  theory  the  following  explanation  of 
these  deviations  should  be  given.  The  muscles  moving 
the  head  of  the  animal  to  the  side  of  the  weaker  illumina- 
tion, having  a  higher  tension  than  their  antagonists,  bring 
about  a  deflection  of  the  animal  toward  the  side  of  the 
weaker  light.  As  soon  as  its  two  photosensitive  areas  in 
the  head — the  animal  has  no  eyes — which  are  not  parallel, 
but  inclined  to  each  other  are  deflected  from  the  perpen- 
dicular upon  the  line  connecting  the  two  lights,  the  photo- 
sensitive areas  of  the  animal  will  no  longer  be  struck  by 
the  lights  at  the  same  angle,  but  on  the  side  of  the  weaker 
light  the  area  will  be  struck  at  an  angle  nearer  to  90°  than 
the  photosensitive  area  exposed  to  the  stronger  light. 
In  this  way  the  change  in  angle  will  compensate  the  dif- 
ference in  intensity  of  the  two  lights  until  the  orientation 
of  the  animal  is  such  that  the  compensation  is  complete 
and  both  photosensitive  areas  receive  the  same  illumina- 
tion. The  animal  will  then  continue  to  move  in  this 
direction. 

Patten  has  computed  the  angle  of  the  photosensitive 
surfaces  for  these  animals  from  the  angle  of  their  orienta- 
tion under  varying  inequalities  of  illumination. 

This  angle  has  been  computed  for  the  blowfly  larva,  using  the  "angu- 
lar deflections  "  already  ascertained.  The  magnitude  of  the  angle  may 
bear  no  direct  relation  to  the  actual  angle  at  which  the  sensitive  areas 
are  located  in  the  body  of  the  animal,  because  of  the  many  factors  which 
may  modify  the  direction  of  the  rays  before  they  fall  on  the  sensitive 
surfaces.  The  significant  test  of  the  hypothesis  would  be  the  constancy 
of  the  angle  when  computed  from  experimental  data  obtained  under 
varying  conditions. 

The  method  of  constructing  such  an 'angle  is  shown  in  Fig.  30,  in 
which  the  opposing  lights  are  assumed  to  be  of  a  two-to-one  ratio  of 
intensity.  The  line  AB  is  drawn  perpendicular  to  the  direction  of  the 
rays  of  light.  On  the  line  AB,  construct  angle  BO C  equal  to  the  actual 
average  angular  deflection  of  the  larva?  at  a  two-to-one  ratio  of  lights. 


80 


TEOPISMS 


The  problem  now  resolves  itself  into  the  construction  of  an  angle  about 
0(7  as  a  bisector,  which  shall  be  of  such  a  magnitude  that  equal  dis- 


Fia.  30. — Diagram  for  constructing  direction  of  motion  of  larvae  under  influence  of  two 
lights  of  different  intensity.     (After  Patten.) 

tances  on  its  opposite  sides  shall  have  projections  on  the  line  AB  of 
the  ratio  of  two  to  one. 

Construction :  From  a  point  D  on  the  line  OC  draw  Dh  perpendicular 


TWO  SOURCES  OF  LIGHT  81 

to  AB.  Lay  off  on  AB  distances  hx  and  hy,  such  that  hy  —  2hx.  From 
x  and  y  erect  lines  perpendicular  to  AB,  they  will  intersect  0(7  at  / 
and  e  respectively.  Bisect  the  line  ef,  and  at  its  middle  point,  g,  con- 
struct a  line  kl  perpendicular  to  OC.  From  the  point  of  intersection  of 
kl  and  yy'  (M),  draw  a  line  to  D.  From  the  intersection  of  kl  and 
xx'  (N),  draw  a  line  to  D. 

The  angle  MDN  is  the  desired  angle. 
Proof:  eg  —  gf  (construction). 
Angle  egM  =  angle  fgN  (construction). 

Angle  Meg  =  angle  Nfg  (alternate  int.  angles  of  parallel  lines, 
yy'  and  xx'  being  parallel  by  construction). 

Therefore  triangle  Meg  =  triangle  Ngf  (side  and  two  adjacent  angles 
being  equal). 

Ng  —  gM  (similar  sides  of  equal  triangles). 

gD  =  gD  (identical). 

Therefore  triangle  NgD  =  triangle  MgD  (rt.  triangles,  altitude  and 
base  equal). 

Therefore  angle  gDM  =  angle  gDN  and  side  DM  =  side  DN. 

Now  by  construction  hx  is  the  projection  of  DN  on  AB  and  hy 
the  projection  of  MD  on  AB,  and  by  construction  hy  =  2hx. 

This  fulfills  all  the  conditions  of  construction. 

The  equal  lines  MD  and  DN  represent  equal  bilateral  sensitive  areas 
inclined  to  each  other  at  such  an  angle,  MDN,  that  the  surface  represented 
by  MD  intercepts  an  area  of  light  twice  as  great  as  the  surface  repre- 
sented by  DN,  its  projection  on  the  perpendicular  to  the  light  rays  being 
twice  as  great  (hy  =  2hx).  But  the  light  falling  on  DN  is  of  twice  the 
intensity  of  the  light  falling  on  DM,  so  that  the  total  amount  of  light 
received  by  each  of  the  equal  areas  is  the  same. 

By  this  method  of  construction,  the  average  angle  of  sensitiveness 
was  computed  for  four  intensity  differences,  using  as  a  basis  the  angular 
deflection  of  the  larva?  as  determined  by  experiment.  The  magnitude 
of  the  angles  is  almost  identical  in  all  four  cases.412 

Experiments  by  a  somewhat  different  method,  to  be 
discussed  in  the  next  chapter,  on  the  positively  heliotropic 
larvae  of  the  barnacle  show  that  these  results  of  Patten 
are  more  general. 

We  may,  therefore,  say  that  the  migration  of  animals 

to  or  from  the  light  is  of  the  nature  of  a  forced  movement 

determined  by  the  effect  of  light  on  the  photosensitive 

elements  of  the  body.    Unequal  illumination  of  symmetri- 

6 


82  TEOPISMS 

cal  photosensitive  elements  on  the  two  sides  of  the  body 
alters  the  tension  of  symmetrical  muscles,  and  as  a  con- 
sequence the  animal  is,  when  moving,  compelled  to  change 
its  direction  of  motion  until  it  is  oriented  in  such  a  way 
to  the  light  that  symmetrical  elements  receive  the  same 
illumination.  In  this  case  the  tension  of  symmetrical 
muscles  is  equal  again  and  the  animal  is  compelled  to 
move  in  this  direction. 

It  has  been  suggested  by  the  anthropomorphic  inter- 
preters of  animal  conduct  that  the  motion  of  an  animal 
to-  a  source  of  light  is  the  same  phenomenon  as  when  a 
human  being  who  has  lost  his  way  in  the  dark  is  attracted 
by  an  illuminated  human  habitation.  As  Bonn  pointed 
out,  the  definite  path  in  which  a  positively  heliotropic 
animal  moves  when  under  the  influence,  of  two  lights, 
shows  that  the  anthropomorphic  interpretation  is  as 
erroneous  in  this  as  in  any  other  case.  A  human  being 
would  go  to  one  of  two  illuminated  houses  and  not  toward 
a  point  between  them,  determined  by  the  relative  intensity 
of  the  two  lights.66 


CHAPTER  IX 

THE  VALIDITY  OF  THE  BUNSEN-EOSCOE  LAW 

FOB  THE  HELIOTEOPIC  EEACTIONS  OF 

ANIMALS  AND  PLANTS 

WE  have  thus  far  said  little  about  the  identity  of  the 
heliotropism  of  plants  and  animals.  Yet  the  two  phe- 
nomena are  essentially  alike.  When  we  keep  positively 
heliotropic  sessile  plants  and  sessile  animals  near  a  win- 
dow, both  will  bend  toward  the  source  of  light,  though 
the  mechanism  of  bending  may  not  be  the  same  in  all 
details,  the  bending  being  produced  in  the  case  of  the  plant 
(and  possibly  in  certain  animals  like  Eudendrium)  by 
unequal  growth  in  length  of  the  plant  on  the  illuminated 
and  shaded  sides;  while  in  the  case  of  higher  animals, 
e.g.,  Spirographis,  it  is  produced  by  differences  in  the 
tension  of  the  muscles  on  the  illuminated  and  shaded 
sides  of  the  animal.  Motile  plant  organisms  like  Volvox, 
are  driven  to  the  source  of  light,  owing  to  differences  in 
the  tension  of  the  contractile  organs  on  the  shaded  and 
illuminated  side,  and  the  same  is  true  for  animals  like 
insects. 

A  further  point  of  coincidence  lies  in  the  validity  of 
the  photochemical  law  of  Bunsen  and  Eoscoe  for  the 
heliotropism  of  animals  and  plants. 

The  law  of  Bunsen  and  Eoscoe  says  that  within  certain 
limits  the  chemical  effect  produced  by  light  increases  in 
proportion  with  the  product  of  intensity  into  the  duration 
of  illumination,  e.g.,  Effect  =  Kit,  where  i  is  intensity,  t 
duration  of  illumination,  and  K  a  constant.  This  is  true 

83 


84 


TEOPISMS 


for  the  blackening  of  photographic  paper  by  light,  and  it 
can  be  shown  that  the  same  law  holds  for  heliotropic 
reactions  of  plants  as  well  as  animals. 

Blaauw46*47  established  this  fact  for  the  etiolated  seed- 
lings of  Avena  saliva.  These  organisms  were  exposed  to 
lights  of  a  definite  candle  power  for  some  time  and  then 
left  in  the  dark.  After  a  certain  time  the  seedlings  began 
to  bend,  becoming  concave  on  that  side  which  had  pre- 
viously been  illuminated.  By  varying  the  candle  power 
of  light  (i)  and  the  duration  of  illumination  (£),  he  found 
that  the  value  of  it  required  to  cause  50  per  cent,  of  the 
seedlings  to  bend  was  always  the  same.  Table  III  gives 

TABLE  III 

Time  required  for  different  intensities  of  light  to  produce  heliotropic  curvatures  in  50  per 
cent,  of  the  seedlings  of  Avena 


Candle-meter 

Duration  of  illumination 

Candle-meter-seconds 

0.00017 

43  hours 

26.3 

0.000439 

13  hours 

20.6 

0.000609 

10  hours 

21.9 

0.000855 

6  hours 

18.6 

0.001769 

3  hours 

19.1 

0.002706 

100  minutes 

16.2 

0.004773 

60  minutes 

17.2 

0.01018 

30  minutes 

18.3 

0.01640 

20  minutes 

19.7 

0.0249 

15  minutes 

22.4 

0.0498 

8  minutes 

23.9 

0.0898 

4  minutes 

21.6 

0.6156 

40  seconds 

24.8 

1.0998 

25  seconds 

27.5 

3.02813 

8  seconds 

24.2 

5.456 

4  seconds 

21.8 

8.453  ' 

2  seconds 

16.9 

18.94 

1  second 

18.9 

45.05 

2/5  seconds 

18.0 

308.7 

2/25  seconds 

24.7 

511.4 

1/25  seconds 

20.5 

1,255 

1/55  seconds 

22.8 

1,902 

1/100  seconds 

19.0 

7,905 

1/400  seconds 

19.8 

13,094 

1/800  seconds 

16.4 

26,520 

I/  1000  seconds 

26.5 

BUNSEN-ROSCOE  LAW 


85 


the  time  required  for  different  intensities  of  light  varying 
from  0.00017  to  26,520  candle  power  to  cause  50  per  cent, 
of  the  seedlings  to  show  heliotropic  curvatures.  As  can 
be  seen,  the  product  it  is  always  approximately  20. 

Ewald  and  the  writer  300> 305  tested  the  validity  of  the 
law  of  Bunsen  and  Roscoe  for  the  heliotropic  curvatures 
of  Eudendrium.  A  number  of  stems  of  Eudendrium, 
from  which  the  polyps  had  been  cut  off,  were  put  upright 
into  a  trough  with  parallel  walls,  containing  sea  water. 
As  soon  as  the  new  polyps  had  regenerated  they  were 
exposed  to  light  of  a  certain  intensity  for  a  short  time 
and  then  kept  in  the  dark.  In  the  dark  the  bending  of 
the  polyps  in  the  direction  of  the  former  source  of  light 
occurred.  The  purpose  was  to  find  the  minimum  time  of 
exposure  required  for  a  given  light  (40  candle  power) 
to  induce  50  per  cent,  of  the  polyps  to  bend  to  the  light 
(Table  IV). 

TABLE  IV 

Percentage  of  polyps  bending  toward  the  former  source  of  light 


Duration  of 

Distance  of  the  polyps  from  the  light  in  meters 

illumination 

0.25 

0.50 

1.00 

1.50 

2.00 

10 

651 

15 

68 

20 

74 

30 

42 

35 

40 

56 

45 

60 

50 

60 

60 

90 

120 

65 

30 

150 

48,  50 

180 

240 

300 

85 

40 

360 

40 

(15) 

420 

57 

1  Very  young,  abnormally  sensitive  polyps. 


86 


•TROPISMS 


If  we  calculate  from  this  the  value  of  the  product  it  for 
different  intensities  of  light  we  find  that  it  obeys  the 
Bunsen-Boscoe  law  (Table  V). 

TABLE  V 


Distance  of  polyps  from 

Time  required  to  call  forth  heliotropic  curvature  in  50  per 
cent,  of  the  polyps 

Observed 

Calculated  according  to  the 
Bunsen-Roscoe  law 

Meters 

0.25 
0.50 
1.00 
1.50 

Minutes 

10 
between  35  and  40 
180 
between  360  and  420 

Minutes 

40 
160 
360 

The  material  varies  considerably  so  that  it  is  not 
always  possible  to  induce  50  per  cent,  of  the  polyps  to 
undergo  heliotropic  curvature.  For  this  reason  Loeb 
and  Wasteneys  312  repeated  these  experiments  by  a  some- 
what different  method. 

We  confined  our  experiments  to  three  intensities  of 
light  by  putting  the  specimens  at  distances  of  25,  37.5, 
and  50  cm.  from  a  Mazda  incandescent  lamp,  of  about  33 
Hefner  candles.  The  times  of  exposure  were  adjusted  so 
that  on  the  assumption  of  the  applicability  of  the  Bunsen- 
Eoscoe  law  the  same  effect,  i.e.,  the  same  percentage  of 
polyps  bending  towards  the  light  should  be  produced. 
Thus  in  some  experiments  the  exposure  for  the  three  dis- 
tances given  was  10,  22.5,  and  40  minutes  respectively, 
in  others,  7,  15.75,  and  28  minutes,  and  so  on.  The  ratios 
of  the  percentage  of  polyps  bending  toward  the  light  for 
the  three  distances  should  be  as  1 : 1 : 1.  Since  the  material 
differed  widely  in  different  experiments  and  in  different 
dishes,  it  was  necessary  to  compute  the  averages  of  a 
large  number  of  experiments. 

The  colonies,  immersed  in  sea  water,  were  arranged 


BUNSEN-KOSCOE  LAW 


87 


in  a  row  in  rectangular  glass  dishes,  the  stems  being  in- 
serted in  holes  made  in  a  layer  of  paraffin  mixed  with  lamp 
black  as  in  the  previous  experiments.  The  rear  side  of 
the  dish  was  also  coated  with  the  paraffin  lamp  black 
mixture  in  order  to  prevent  reflection  of  light  from  the 
slightly  uneven  back  surface  of  the  dish. 

Table  VI  gives  a  summary  of  the  results.    The  first 
three  columns  give  the  times  of  exposure  for  the  three 

TABLE  VI 


Times  of  exposure  in  minutes 

Ratio  of  per  cent,  of  hydranths  bending 
towards  light 

25  cm. 

37.5  cm. 

50  cm. 

25  cm.:  37.5  cm. 

25cm.:  50  cm. 

37.5  cm.:  50  cm. 

15 

60 

1.50 

20 

80 

1.30 

10 

22.5 

40 

1.20 

(3.08) 

(2.56) 

10 

22.5 

40 

0.94 

1.47 

1.55 

10 

22.5 

40 

1.57 

(2.30) 

(2.43) 

10 

22.5 

40 

1.43 

1.04 

0.94 

10 

22.5 

40 

0.76 

1.09 

1.47 

10 

22.5 

40 

1.05 

1.13 

0.90 

0.96 

10 

22.5 

40 

1.15 

0.99 

7 

15.75 

28 

0.84 

0.62 

0.74 

7 

15.75 

28 

1.70 

0.49 

0.58 

7 

15.75 

28 

0.85 

1.25 

1.35 

7 

15.75 

28 

(2.09)1 

0.99 

1.08 

7 

15.75 

28 

1.14 

1.15 

0.55 

7 

15.75 

28 

0.44 

0.92 

0.44 

7 

15.75 

28 

1.52 

0.80 

0.61 

7 

15.75 

28 

0.59 

0.36 

0.70 

7 

15.75 

28 

0.48 

1.07 

0.31 

7 

15.75 

28 

1.00 

0.48 

1.80 

7 

15.75 

28 

0.69 

1.09 

0.81 

7 

15.75 

28 

1.26 

0.85 

1.09 

7 

15.75 

28 

0.86 

1.38 

0.85 

7 

15.75 

28 

0.70 

1.07 

1.59 

7 

15.75 

28 

0.77 

1.24 

7 

15.75 

28 

0.60 

Mean  

1.02 

0.99 

1.02 

Probable  error  .  . 

±0.01 

±0.01 

±0.01 

1  Bracketed  values  being  extreme  variates  are  excluded  from  calculations  of  the  means 
and  probable  errors. 


88  TKOPISMS 

distances  of  the  source  of  light,  selected,  as  stated,  on  the 
assumption  that  the  Bunsen-Eoscoe  law  holds.  On  that 
assumption  the  ratio  of  percentage  bent  in  any  two  or  all 
three  dishes  on  any  one  day  should  equal  1.0.  These  ratios 
for  each  pair  of  distances  of  the  source  of  light  are 
given  in  the  three  other  columns  of  the  table.  The  per- 
centage bending  was  only  compared  in  dishes  containing 
material  regenerated  and  exposed  on  any  one  day,  since 
only  in  this  case  was  there  any  likelihood  that  the  material 
was  in  any  way  uniform,  and  since  only  in  this  case  the 
experiments  were  carried  on  at  the  same  temperature  and 
the  same  conditions  of  regeneration. 

The  result  was  that  the  observed  ratios  were  as 
1.02 : 0.99 : 1.02  (with  a  probable  error  of  =t  0.01)  while  the 
values  calculated  on  the  assumption  of  the  validity  of 
the  Bunsen-Eoscoe  law  were  as  1:1:1;  i.e.,  the  results 
showed  as  great  an  approximation  between  observed  and 
calculated  values  as  one  could  expect. 

There  is  a  second  method  for  testing  the  validity  of 
the  Bunsen-Eoscoe  law,  based  on  the  use  of  two  sources  of 
light  of  equal  intensity. 

If  it  is  true  that  the  heliotropic  efficiency  of  light  is 
determined  by  the  product  of  intensity,  i,  into  duration  of 
illumination,  t,  we  can  alter  this  product  by  varying  t  as 
well  as  by  varying  i. 

Eadl  had  shown  that  the  position  of  the  eye  of  the 
fresh  water  crustacean,  Daphnia,  is  determined  by  the 
position  of  a  source  of  light,447  and  Ewald 145  found 
that  by  exposing  the  eye  to  two  different  sources  of  light 
simultaneously  the  eye  is  put  into  a  position  determined 
by  the  relative  intensity  of  the  two  lights.  When  one  light 
remained  constant  and  the  intensity  of  the  other  light 
was  lowered  the  position  of  the  eye  changed.  He  now 


BUNSEN-BOSCOE  LAW  89 

could  show  that  when  the  duration  of  illumination  of  one 
eye  was  altered  by  a  rotating  opaque  disk  with  one  sector 
cut  out,  the  heliotropic  effect  on  the  eye  of  Daphnia  was 
the  same  as  when  the  intensity  i  of  the  same  light  was 
reduced  to  an  amount  corresponding  to  the  Bunsen- 
Eoscoe  law. 

Under  the  influence  of  two  constant  lights  of  equal 
intensity  heliotropic  animals  move  in  a  direction  at  right 
angles  to  the  line  connecting  the  two  lights.  If  the  law  of 
Bunsen  and  Eoscoe  holds  the  effect  of  a  constant  light 
should  be  diminished  if  a  rapidly  rotating  opaque  disk 
with  one  sector  cut  out  be  put  in  front  of  the  light,  and  the 
diminution  should  be  equal  to  the  fraction  of  the  arc  of 
the  sector.  Thus  a  sector  of  90°,  which  reduces,  the  total 
duration  of  illumination  to  one-fourth,  should  also  reduce 
the  heliotropic  effect  of  the  light  to  one-fourth,  and  the  ani- 
mal should  deviate  from  the  old  direction  in  the  direction 
toward  the  light  without  a  disk  before  it.  If,  however,  we 
lower  the  intensity  of  the  latter  light  to  one-fourth  by 
doubling  its  distance  we  also  reduce  its  heliotropic  effect 
to  one-fourth,  and  now  the  animal  should  move  again  in  a 
line  at  right  angles  to  the  line  connecting  the  two  lights. 

The  following  experiments  carried  out  by  Loeb  and 
Northrop 309  on  the  larvae  of  the  barnacle  are  perhaps  the 
best  proof  for  the  validity  of  the  Bunsen-Eoscoe  law  for 
animal  heliotropism. 

These  animals  are  small  and  can  be  obtained  in  large  numbers.  They 
were  made  to  collect  in  the  corner  of  a  dish  with  a  little  sea  water  and 
were  then  sucked  up  into  a  pipette  ef,  Fig.  31,  which  was  blackened  with 
the  exception  of  the  opening.  When  such  a  pipette  is  put  into  a  glass 
dish  with  parallel  walls  whose  bottom  is  black  (by  putting  paraffin  black- 
ened with  lampblack  at  the  bottom  of  the  dish)  the  larvae  will  flow  out  in 
a  fine  stream  and  swim  when  they  are  positively  heliotropic  in  a  straight 
line  toward  the  source  of  light.  They  thus  form  a  rather  narrow  white 
trail  on  the  dark  bottom  and  it  is  possible  to  measure  the  angle  of  this 


90 


TEOPISMS 


trail  with  the  line  connecting  the  two  lights.  In  this  way  in  each  observa- 
tion the  average  trail  of  thousands  of  individuals  is  measured.  By  using 
one  constant  and  one  intermittent  source  of  light  and  comparing  the 
results  with  those  obtained  by  two  constant  lights  we  can  test  the  validity 
of  the  Bunsen-Roscoe  law. 

The  method  of  the  experiments  was  as  follows :  abed  (Fig.  31)  is  a 
square  dish  of  optical  glass  with  blackened  bottom  and  containing  a 


FIG.  31. — Method  for  the  proof  of  the  validity  of  Bunsen-Roscoe  law  for  the  positively 
heliotropic  larvae  of  the  barnacle.     (After  Loeb  and  Northrop.) 

layer  of  sea  water.  A  and  B  are  two  lights,  the  intensity  of  which  is 
determined  by  a  Lummer-Brodhun  contrast  photometer.  In  front  of 
each  light  is  a  screen  with  a  round  hole  permitting  a  beam  .of  light  to 
go  to  the  dish.  The  lights  and  the  dish  abed  are  so  adjusted  that  the 
two  beams  of  light  striking  the  sides  ab  and  be  at  right  angles  cross 
each  other  in  the  middle  of  the  dish.  The  light  A  is  fixed  while  the 
light  B  is  movable  on  an  optical  bench.  The  experiment  is  made  in 


BUNSEN-KOSCOE  LAW 


91 


a  dark  room  and  the  lights  A  and  B  are  enclosed  in  a  box.  At  the 
beginning  of  the  experiments  the  pipette  is  filled  with  a  dense  suspension 
of  larvae  in  sea  water  and  then  put  with  its  point  touching  the  bottom 
of  the  dish.  The  animals  flow  out  in  a  fine  stream  which  is  narrow  at 
the  opening  of  the  pipette  and  widens  slightly,  owing  probably  to  the 
negative  stereotropism  of  the  animals. 
A  glass  plate  (Fig.  32)  tiikl,  which  has 
a  strong  red  line  no  and  a  fine  parallel 
line  pq  (cut  with  a  diamond),  is  then  put 
on  the  dish  and  so  adjusted  that  pq  is  in 
the  middle  of  the  stream  fg  of  the  ani- 
mals. Then  the  angle  a  which  pq  makes 
with  the  perpendicular  from  A  on  ab  is 
measured.  This  perpendicular  is  marked 
in  the  form  of  a  red  line  on  the  black 
base  on  which  the  glass  vessel  abed 
stands.  The  angle  a  is  measured  with  a 
goniometer.  When  the  lights  are  equal 
in  intensity  a  should  be  45° ;  if  the  two  lights  have  different  intensities 
and  if  A  be  the  stronger  light  a  should  become  smaller  with  increas- 
ing difference  in  intensity.  The  individual  measurements  vary  com- 
paratively little,  as  long  as  the  difference  in  the  intensity  of  the  two 
lights  is  not  too  great;  for  this  reason  our  observations  do  not  go 
beyond  a  wider  ratio  of  the  two  lights  than  10 : 1,  though  4 : 1  is 
perhaps  the  limit  for  good  results.  Table  VII  gives  the  results.  A 
is  always  the  stronger  light.  Each  table  is  the  average  of  from  40  to  60 
individual  observations,  each  being  the  average  of  the  path  of  many 
thousands  of  animals. 

TABLE  VII 


FIG.  32. 


Value  of  a  for  different  ratios  of  intensities  of 
the  two  lights 


Ratio  of 

the  two  lights  . 

1:  1 

2:1 

4:  1 

10:  1 

Value  of 

a  (direction  of 

path)  

45.6° 

40° 

34.4° 

28.8° 

In  the  next  series  of  experiments  an  opaque  rotating  disk  with  one 
sector  cut  out  was  placed  before  light  B.  In  one  set  of  experiments  the 
sector  cut  out  was  90°.  The  rate  of  rotation  (by  an  electric  motor) 
was  1,500  to  2,500  revolutions  per  minute.  The  other  light  was  constant 
and  its  distance  was  chosen  on  the  assumption  of  the  validity  of  the 
Bunsen-Roseoe  law  for  these  cases.  Thus  when  the  two  lights  without 
sector  were  equal  at  a  given  distance  of  A}  by  putting  90°  sector  before 


92 


TKOPISMS 


B,  it  was  assumed  that  the  ratio  of  effects  would  be  the  same  as  if,  with 
constant  light,  B  had  been  placed  at  the  double  distance  and  the  ratio 
of  intensities  of  the  two  lights  had  been  4 : 1.  Going  on  such  a  calculation 
we  should  expect  the  same  values  for  a  as  in  Table  VII. 

As  one  sees  from  Table  VIII,  the  observed  values  are  slightly  smaller 
but  practically  identical  with  the  values  obtained  when  the  two  lights 
are  constant.  The  deviation  is  probably  due  to  the  well  established 
fact  that  the  photochemical  efficiency  of  an  intermittent  light  is  a  trifle 
less  than  that  calculated  on  the  basis  of  the  Bunsen-Roscoe  law. 

TABLE  VIII 


Ratio 

of 

the 

two 

lights  

1 

1 

2:  1 

4: 

1 

Value 

of 

a.  . 

44 

9° 

383° 

34. 

1° 

Value  of  a  when  one  light  is  inter- 
mittent (90°  sector)  and  the 
othar  constant,  and  the  efficiency 
of  the  two  lights  is  calculated  on 
the  basis  of  the  validity  of  the 
Bunsen-Roscoe  photochemical 
law 


We  carried  out  some  experiments  with  a  sector  of  144°.  When  the 
efficiency  of  both  lights  was  equal  on  the  assumption  of  the  validity 
of  the  Bunsen-Roscoe  law  a.  was  found  to  be  44.9°  (instead  of  45°),  and 
for  the  ratio  2 : 1  a  was  found  to  be  38.8°.  The  values  are,  within  the 
limits  of  error,  identical  with  the  values  in  Tables  VII  and  VIII.309 

Bradley  M.  Patten  also  showed  that  for  the  heliotropic 
reactions  of  the  negatively  heliotropic  larvae  of  the  fly 
the  law  of  Bunsen  and  Eoscoe  holds. 

Photochemical  processes  have  a  very  small  tempera-  j 
ture  coefficient  and  it  agrees  with  this  that  lowering  of 
temperature  within  the  limits  compatible  with  the  motility  j 
of  animals  does  not  affect  the  heliotropic  response ;  on  the 
contrary,  we  shall  see  that  in  certain  crustaceans  (e.g., 
DapJinia)  lowering  of  the  temperature  may  enhance  posi- 
tive heliotropism.296 

We  must,  therefore,  conclude  that  the  light  produces 
in  an  eye  or  an  element  of  the  photosensitive  skin  a  chemi- 


BUNSEN-KOSCOE  LAW  93 

cal  reaction  which  results  in  the  formation  of  a  certain 
mass  of  a  reaction  product.  This  mass  acts  on  the  periph- 
eral nerve  endings  and  brings  about  an  as  yet  unknown 
change  in  the  brain  elements  with  which  these  nerve  end- 
ings are  connected.  This  change  in  turn  affects  the  tone 
or  tension  of  the  muscles  with  which  the  brain  elements 
are  connected.  When  the  rate  of  photochemical  reaction 
is  the  same  in  both  eyes  or  in  the  photosensitive  elements 
on  both  sides  of  the  body,  the  change  of  tone  in  the  sym- 
metrical muscles  of  both  sides  of  the  body  is  the  same 
and  no  change  in  the  position  or  direction  of  motion  of  the 
organism  should  occur.  If  the  rate  of  illumination  is  dif- 
ferent in  both  eyes,  differences  in  the  relative  tension  of 
the  symmetrical  muscles  occur,  which  make  the  motion 
to  the  source  of  light  easy  and  in  the  opposite  direction 
more  difficult  when  the  animal  is  positively  heliotropic. 
For  the  negatively  heliotropic  animal  the  opposite  effect 
will  be  brought  about. 

These  experiments,  therefore,  show  that  the  tropism 
theory  not  only  allows  us  to  predict  the  nature  of  the  ,  / 
animal  reactions  but  allows  us  to  predict  them  quantita-  " 
tively.    Thus  far  the  tropism  theory  is  the  only  one  which 
satisfies  this  demand  of  exact  science. 

The  degree  of  directness  with  which  a  heliotropic  ani- 
mal goes  to  or  from  a  source  of  light  depends,  aside  from 
the  degree  of  perfection  of  its  locomotor  apparatus,  upon 
the  intensity  of  the  light  and  the  relative  sensitiveness 
of  the  animal.  Animals  which  in  strong  light  will  move 
in  approximately  straight  lines  to  or  from  the  source  of 
light  may  in  weak  light  reach  their  goal  in  a  more  or  less 
irregular  zigzag  line.  This  is  easily  understood.  When 


94  TEOPISMS 

an  animal  by  chance  gets  its  median  plane  too  far  out 
of  the  direction  of  the  rays  of  light  (we  assume  them  to 
be  parallel),  the  rate  of  photochemical  reaction  will  be- 
come different  in  both  eyes.  As  soon  as  the  difference 
between  the  photochemical  reaction  products  in  both  eyes 
exceeds  a  certain  limit  the  animal  will  automatically  put 
its  plane  of  symmetry  again  into  the  direction  of  the  rays 
of  light.  The  weaker  the  light  and  the  less  sensitive  the 
animal,  the  longer  it  will  take  until  this  happens,  and  the 
greater  the  freedom  of  the  animal  to  deviate  from  the 
straight  line. 


CHAPTER  X 

THE  EFFECT  OF  EAPID  CHANGES  IN  INTEN- 
SITY OF  LIGHT 

IT  may  prove  necessary  to  make  a  similar  assumption 
for  the  effect  of  a  constant  illumination  as  was  made  by 
Nernst  for  the  theory  of  the  action  of  galvanic  currents, 
namely  that  there  are  two  antagonistic  processes  going 
on,  one  being  the  photochemical  effect  of  light  and  the 
second  either  a  process  of  diffusion  of  the  substances 
formed  or  a  chemical  reaction  of  the  opposite  character 
as  that  caused  by  the  action  of  the  light.  Many  animals 
which  are  oriented  by  constant  illumination  react  by  a 
quick,  jerky  movement  when  the  intensity  of  light  is 
either  rapidly  increased  or  diminished.  In  this  case  the 
effect  is  determined  by  the  rapidity  of  the  change  in  the 
intensity,  ^  ,  and  not  by  the  product  of  intensity  into  dura- 
tion of  illumination,  it.297  These  twitching  or  jerking 
effects  caused  by  a  rapidly  changing  intensity  of  light 
are  comparable  to  the  twitching  brought  about  in  a  muscle 
by  a  rapid  increase  or  decrease  in  the  intensity  of  a  cur- 
rent. The  writer  described  such  reactions  first  for  tube 
worms  like  Serpula,  which  withdraws  suddenly  into  its 
tube  when  a  shadow  passes  over  it  or  when  the  intensity 
of  light  is  suddenly  diminished  in  some  other  way.  The 
anthropomorphists,  of  course,  declare  this  reaction  to  be 
induced  by  the  instinctive  fear  of  an  enemy,  oblivious  of 
the  fact  that  if  they  were  consistent  they  would  have  to 
give  the  same  explanation  for  the  twitching  of  a  muscle 
upon  rapid  changes  in  the  intensity  of  a  current.  The 

95 


96 


TROPISMS 


problem  to  be  solved  is  in  both  cases  a  purely  physico- 
chemical  one.  It  was  also  found  that  the  motions  of 
certain  animals  stop  when  they  come  suddenly  from  strong 
light  into  weak  light.  This  was  observed  in  planarians 
which  as  a  consequence  collect  in  greater  density  in  spots 

°f  the  space  where  the  intensity  of 

light  is  a  relative  minimum.291  The 
difference  in  the  conduct  of  helio- 
tropic  organisms  like  Daphnia  which 
go  to  or  from  the  light  and  animals 
like  planarians  which  come  to  rest 
where  the  intensity  of  light  is  a  rela- 
tive minimum  can  be  demonstrated 
by  putting  them  into  a  circular  vessel 
( Fig.  33 ) .  The  positively  heliotropic 
animals  collect  at  a,  the  negatively 
heliotropic  at  &,  while  the  planarians 
collect  at  c  and  d  where  the  intensity 
of  light  is  a  minimum.  Reactions  de- 
termined by  the  value  Jj  do  not  lead 
to  phenomena  of  orientation,  though 
such  (improperly  called)  "fright 
reactions  "a  occur  in  many  helio- 
tropic animals;  they  may  lead,  however,  to  collections 
of  animals. 

Jennings  has  maintained  that  all  reactions  of  unicel- 
lular organisms  are  due  to  "fright"  or  "avoiding  reac- 

aThe  reader  should  notice  the  difference  in  tlie  treatment  of  animal 
conduct  from  the  point  of  view  of  the  physicist  and  of  the  introspective  psy- 
chologist. What  the  physicist  expresses  correctly  by  the  term  —  the  an- 
thropomorphic biologist  explains  in  terms  of  human  analogy  as  "  avoiding 
reaction  "  or  "  fright  reaction,"  a  term  which  not  only  assumes  the  existence 
of  sensations  without  any  adequate  proof,  but  removes  the  problem  from  the 
field  of  quantitative  experimentation. 


FIG.  33. — Difference 
in  place  of  gathering  be- 
tween heliotropic  animals 
and  animals  which  come 
to  rest  when  reaching  a 
relative  minimum  in  the 
intensity  of  light.  In  a 
circular  vessel  a  c  b  d  and 
W  W  representing  the 
window,  positively  helio- 
tropic animals  will  collect 
at  a,  negatively  heliotropic 
animals  at  6,  and  animals 
which  come  to  rest  where 
the  intensity  of  light  is  a 
relative  minimum  at  c 
and  d. 


CHANGES  IN  INTENSITY  97 

tions,"  and  it  seems  as  if  at  one  time  lie  even  intended 
to  deny  the  existence  of  tropisms  and  to  maintain  that  all 
animals  were  influenced  only  by  rapidly  changing  intensi- 
ties of  light.  It  is  needless  to  discuss  such  an  idea  (which 
he  probably  no  longer  holds)  in  view  of  the  contents  of  the 
preceding  chapters.  He  seems,  however,  to  cling  to  it  as 
far  as  asymmetrical  unicellular  organisms  are  concerned. 
When  moving  about  Paramcecia  often  reverse  the  direc- 
tion of  their  progressive  motion  for  a  moment,  but  then 
do  not  return  in  the  old  direction,  moving  sidewise,  on 
account  of  the  asymmetry  in  the  arrangement  of  their 
cilia.  Jennings  is  probably  right  in  assuming  that  this 
factor  can  lead  to  collections  of  such  infusorians,  since 
it  may  prevent  their  leaving  a  drop  and  going  into  the  sur- 
rounding medium.  When,  e.g.,  at  the  boundary  of  the  two 
media  such  a  reversal  of  the  action  of  the  cilia  occurs,  the 
organisms  are  prevented  from  crossing  from  one  medium 
into  the  other. 

But  Jennings  goes  too  far  in  this  attempt,  when  he 
tries  to  explain  the  heliotropic  reactions  of  certain  uni-i 
cellular  organisms,  e.g.,  Euglena,  in  this  way.  He  main- 
tains 253  that  unicellular  organisms  like  Euglena  go  to  the 
light  on  account  of  shock  movements  produced  by  the 
shading  of  the  photosensitive  region  of  the  animal. 
Euglena  moves  with  a  constant  rotation  around  its  longi- 
tudinal axis  and  Jennings  assumes  that  in  a  certain  phase 
of  the  rotation  a  photosensitive  element  (the  eye  spot) 
of  the  organism  is  shaded.  This  he  thinks  causes  a  shock 
movement,  whereby  the  animal  is  swerved  to  the  light 
again  during  the  next  half  of  the  spiral  revolution,  and 
so  on.  Similarly  in  negatively  heliotropic  Euglena  the 
swerving  away  from  the  light  is,  according  to  Jennings, 
the  shock  movement  caused  by  the  increased  illumination 


98  TROPISMS 

of  the  photosensitive  end  of  the  animal  produced  by  swerv- 
ing toward  the  light  during  the  previous  half  of  the  spiral 
revolution.  Bancroft 21  showed  that  Jennings 's  theory 
was  based  upon  incomplete  facts. 

According  to  Jennings's  view  positive  heliotropism  is  conditioned  by 
and  should  be  accompanied  by  shock  movements  produced  by  sudden 
shading  (—shading  reaction),  and  negative  heliotropism  should  always 
be  accompanied  by  shock  movements  produced  by  sudden  illumination. 

It  has  been  found,  however,  that  this  usual  association  of  shock 
movements  with  tropism  is  not  a  necessary  one,  but  that  it  can  be 
destroyed  if  the  proper  means  be  taken.  Consequently  the  view  that  the 
heliotropic  swerving  is  a  shock  movement  must  fall. 

When  Euglence  from  Culture  B  were  placed  in  the  rays  of  the  arc 
light,  at  a  distance  of  four  or  five  feet  from  the  light,  they  were  strongly 
positively  heliotropic  and  gave  the  shading  reaction.  When,  however, 
they  were  gradually  brought  nearer  to  the  light  a  point  was  reached 
at  which  the  heliotropism  disappeared  but  the  shading  reaction  per- 
sisted. When  moved  still  closer  to  the  light  they  became  negatively 
heliotropic  but  still  without  any  change  of  the  shading  reaction.  When 
moved  still  closer  to  the  light,  there  was  a  short  time  when  no  shock 
movements  could  be  obtained,  but  soon  the  illumination  reaction  ap- 
peared. At  the  same  time  the  negative  heliotropism  became  more  prompt 
and  precise.  Finally,  when  the  light  was  still  further  increased  and 
allowed  to  act  for  a  considerable  time,  even  the  illumination  reactions 
frequently  disappeared  completely,  and  a  most  pronounced  and  compell- 
ing negative  heliotropism  held  full  sway.  .  .  . 

It  is  very  evident,  then,  that  the  invariable  correlation  of  positive 
heliotropism  with  the  shading  reaction,  which  is  required  by  Jennings's 
theory,  does  not  exist.     Both  kinds  of  heliotropism  may  be  associated 
with  either  the  shading  or  the  illumination  reaction.     Accordingly,  it 
must  be  concluded  that  the  heliotropic  mechanism  does  not  depend  upon  J 
the  mechanism  for  the  shock  movements,  but  that  the  two  mechanisms    ! 
are  independent.21 

The  simplest  method  of  determining  whether  or  not 
the  orientation  of  flagellates  depends  upon  rapid  changes 
in  intensity  of  light  or  upon  constant  illumination  can  be 
furnished  with  the  aid  of  intermittent  light.  We  know 
that  a  striped  muscle  contracts  only  when  a  current  is 


CHANGES  IN  INTENSITY  99 

made  or  broken,  but  not  while  the  constant  current  lasts. 
Henc6  a  rapidly  alternating  current  throws  the  muscle 
into  tetanus,  while  the  constant  current  has  no  effect. 
If  it  is  the  rapid  change  in  the  intensity  of  light  which 
causes  the  swimming  of  a  positively  heliotropic  Euglena 
to  the  light,  an  intermittent  light,  of  a  sufficient  number  of 
alternations  per  second,  should  be  much  more  efficient 
than  a  constant  light;  while  in  case  the  positive  helio- 
tropism  is  determined  by  constant  illumination,  this 
should  not  be  the  case  and  the  Bunsen-Eoscoe  law  should 
hold. 

Mast348  has  recently  published  experiments  on  the 
relative  efficiency  of  the  various  parts  of  the  spectrum 
by  a  method  based  on  the  assumption  of  the  validity  of 
the  Bunsen-Eoscoe  law  for  the  heliotropic  orientation  of 
these  organisms.  If  his  assumption  b  is  correct,  it  con- 
tradicts the  theory  which  Jennings  and  Mast  have  de- 
fended now  for  more  than  fifteen  years ;  if  his  assumption 
is  wrong,  his  experiments  on  the  relative  efficiency  of 
various  parts  of  the  spectrum  cannot  be  correct.  Since, 
however,  Mast's  results  with  this  method  coincide  with 
those  by  Loeb  and  Wasteneys312  obtained  by  a  direct 
method,  it  is  very  probable  that  the  law  of  Bunsen  and 
Roscoe  holds  for  the  heliotropic  reactions  of  Euglena  and 
unicellular  flagellates  in  general,  and,  if  this  is  true,  the 
heliotropic  reactions  of  unicellular  algae  (Euglena  in- 
cluded) are  determined  by  light  of  constant  intensity. 

t>  He  does  not  seem  to  have  noticed  that  his  method  was  based  on  this 
assumption. 


CHAPTER  XI 

THE  EELATIVE  HELIOTBOPIC  EFFICIENCY  OF 
LIGHT  OF  DIFFEBENT  WAVE  LENGTHS 

1.  The  validity  of  the  Bunsen-Boscoe  law  for  the  helio- 
tropic  reactions  of  animals  and  plants  leaves  no  doubt 
that  these  reactions  are  determined  by  the  rate  of  photo- 
chemical processes. /Heliotropic  reactions  depend,  how- 
ever, not  only  upon  the  intensity  but  also  upon  the  wave 
length  of  light.  Photochemistry  shows  that  the  most 
efficient  wave  length  varies  with  the  nature  of  the  photo- 
chemical substance  and  that  comparatively  slight  changes 
in  the  constitution  of  a  molecule  may  bring  about  con- 
siderable changes  in  the  relative  efficiency  of  different 
wave  lengths.  The  search  for  differences  in  the  helio- 
tropic  effect  of  different  wave  lengths  can  be  of  service 
in  detecting  the  nature  of  the  photochemical  substances 
responsible  for  heliotropic  reactions. 

The  investigations  on  the  relative  heliotropic  efficiency 
of  different  wave  lengths  have  generally  been  undertaken 
for  a  different  purpose,  namely,  to  get  information  con- 
cerning the  color  sensations  of  animals.  Graber  gave 
it  as  the  result  of  his  observations  that  all  animals  which 
were  fond  of  light  were  also  fond  of  blue,  and  animals 
which  were  fond  of  dark  were  also  fond  of  red.180  He  put 
animals  into  a  box  half  of  which  was  covered  with  trans- 
parent glass  and  half  with  an  opaque  object,  and  then 
counted  the  relative  numbers  of  organisms  in  both  halves 
of  the  box.  He  then  replaced  these  screens  by  colored 
glasses  and  obtained  the  above-mentioned  result.  The 
100 


WAVE  LENGTH 

writer  showed  that  the  animals  are  neither  fond  of  blue 
nor  of  red  but  are  oriented  by  the  light  in  the  same  way 
as  are  plants,  and  the  statement  that  animals  which  were 
"fond"  of  light  also  were  "fond"  of  blue  and  those 
which  were  "fond"  of  "dark"  were  "fond"  of  red 
the  writer  explained  in  a  simpler  way,  namely  that  the 
light  filtered  through  red  glass  had  a  smaller  orienting 
effect  than  the  light  filtered  through  blue  glass.287  Hence 
red  glass  acted  like  an  opaque,  blue  glass  like  a  trans- 
parent screen.  This  had  already  been  known  to  be  true 
for  the  heliotropic  reactions  of  plants  for  which  Sachs 
had  shown  that  they  occur  behind  a  blue  glass  in  the  same 
way  as  behind  common  window  glass,  while  behind  red 
glass  heliotropic  reactions  do  not  occur  at  all  or  occur 
very  slowly  as  if  the  light  were  weak.  The  writer  was 
able  to  show  that  the  same  is  true  for  animals.267  When 
positively  heliotropic  animals  are  put  into  a  box  covered 
with  blue  glass  they  go  as  rapidly  to  the  window  side 
as  when  the  box  is  uncovered;  while  when  it  is  covered 
with  red  glass  the  animals  will  go  to  the  window  but  more 
slowly  and  irregularly.  Behind  a  red  screen  they  behave 
therefore  as  if  they  were  exposed  to  weak  light. 

Blue  glass  is  permeable  not  only  for  blue  but  also  for 
rays  which  produce  the  sensation  of  green.  Paul  Bert 38 
had  already  made  experiments  with  positively  heliotropic 
Daphnia  in  a  solar  spectrum  and  found  that  the  animals 
"accouraient  beaucoup  plus  rapidement  au  jaune  ou  au 
vert  qu'  a  toute  autre  couleur."a  Bert  concluded  from 
this  that  to  the  eye  of  a  Daphnia  those  parts  of  the  spec- 
trum appear  brightest  which  also  appear  brightest  to  the 
human  eye.  Bert's  aim  was  to  find  out  whether  the  sensa- 

a  This  method  of  ascertaining  the  most  efficient  part  of  the  spectrum  is 
not  reliable  and  has  been  replaced  by  other  methods. 


102  TEOPISMS 

tions  caused  by  light  in  lower  animals  are  the  same  as 
those  caused  in  a  human  being.  But  even  if  the  relative 
efficiency  of  the  various  parts  of  the  spectrum  were  the 
same  for  the  sensations  of  brightness  in  human  beings  and 
for  the  heliotropic  reactions  in  lower  animals,  it  would  not 
prove  that  the  latter  also  have  sensations  of  brightness. 
For  we  have  no  guarantee  that  the  heliotropic  reactions 
of  lower  animals  are  due  to  or  accompanied  by  sensations 
of  brightness.  If  the  yellow-green  rays  are  the  most 
efficient  in  causing  heliotropic  reactions  in  an  organism, 
it  suggests  only  that  in  such  an  organism  the  photosensi- 
tive substance  responsible  for  the  heliotropic  response  is 
most  easily  decomposed  by  the  yellow-green  part  of 
the  spectrum. 

A  similar  error  of  reasoning  as  that  by  Bert  has 
recently  been  made  by  Hess.  Hess  corroborated  what  the 
writer  had  already  pointed  out,  that  the  red  rays  of  the 
visible  solar  spectrum  are  the  least  efficient,  and  he  found, 
moreover,  as  Bert  had  found  for  Daphnia,  that  for  the 
heliotropic  reactions  of  a  number  of  animals,  from  the 
fishes  down,  the  yellow-green  region  of  the  solar  spectrum 
is  the  most  efficient.  Now  it  happens  that  to  a  totally 
color  blind  human  being,  to  whom  the  different  parts  of 
the  solar  spectrum  appear  only  as  shades  of  gray,  the 
region  A  =  540  w  in  the  yellow-green  appears  to  be  the 
brightest;  while  the  red  part  of  the  spectrum  gives  a 
very  faint  sensation  of  brightness.  From  this  similarity 
or  apparent  identity  between  the  relative  effects  of  dif- 
ferent wave  lengths  upon  the  heliotropic  effects  of  certain 
lower  animals  and  upon  the  sensations  of  brightness  of 
a  totally  color  blind  human  being,  Hess  draws  the  con- 
clusion that  these  animals  are  totally  color  blind.  In  our 
opinion  the  only  conclusion  which  Hess  has  a  right  to 


WAVE  LENGTH  103 

draw  is  that  the  photosensitive  substance  which  causes 
sensations  of  brightness  in  the  eye  of  the  color  blind 
human  being  is  either  identical  with  or  is  affected  in  a 
similar  way  by  light  waves  as  is  the  substance  giving 
rise  to  heliotropic  reactions  in  certain  animals.  This 
assumption  is  entirely  adequate  and  harmonizes  better 
with  the  facts  than  the  assumption  made  by  Hess.  The 
substance  responsible  for  the  sensations  of  brightness  in 
the  eyes  of  the  totally  color  blind  human  being  is  visual 
purple  which  is  bleached  most  rapidly  by  light  of 
A  =  540  w.  That  our  objection  is  justified  is  proved  by 
the  experiments  of  v.  Frisch  on  bees. 

Frisch168  has  shown  by  very  ingenious  and  care- 
ful experiments  that  bees  can  be  trained  to  discriminate 
between  blue  and  yellow  but  not  between  different  shades 
of  gray.  On  a  table  were  put  square  cardboards  of  dif- 
ferent shades  of  gray  and  among  them  one  blue  piece  of 
cardboard.  On  each  gray  square  was  put  a  watch  crystal 
containing  water,  while  the  watch  crystal  on  the  blue  con- 
tained sugar  water.  The  bees  of  an  observation  hive 
visiting  the  sugar  crystal  were  marked  with  a  fine  paint 
brush.  After  a  sufficient  period  of  training  it  was  found 
that  the  marked  bees  always  went  directly  to  those  crys- 
tals which  were  on  a  blue  piece  of  cardboard,  whether  they 
contained  sugar  water  or  pure  water;  and  when  there 
was  no  sugar  water  on  the  blue  cardboard  they  alighted 
on  any  blue  object,  e.g.,  a  blue  pencil.  The  crystals  and 
cardboard  pieces  were  always  renewed  in  different  tests, 
to  avoid  any  influence  of  odor.  It  was  never  possible  to 
train  the  bees  to  select  a  piece  of  cardboard  with  a  definite 
shade  of  gray  among  cardboards  of  different  shades  of 
gray. 

Hess  had  shown  that  for  the  heliotropic  reaction  of 


104  TEOPISMS 

bees  the  yellowish-green  part  of  the  spectrum  is  most 
efficient,  and  he  concluded  that  bees  are  totally  color  blind. 
To  a  totally  color  blind  person  the  blue  cardboard  appears 
only  like  a  shade  of  gray,  and  such  a  person  is  unable  to 
learn  to  discriminate  between  a  blue  and  gray  piece  of 
cardboard,  v.  Frisch's  experiments  support  the  con- 
clusion that  it  is  unjustifiable  to  use  experiments  on 
heliotropism  to  draw  conclusions  concerning  light  or 
color  sensations,  v.  Frisch  and  Kupelwieser 169  have 
also  demonstrated  selective  effects  of  different  light 
waves  for  Daphnia  which  differ  from  those  found  for  the 
eye  of  a  totally  color  blind  person,  and  their  observations 
have  been  confirmed  by  Ewald.147 

2.  Hess's  conclusions  are  in  conflict  with  another 
group  of  facts.  For  many  plants  the  blue  region  of  the 
solar  spectrum  is  the  most  efficient.  Hess  is,  therefore, 
compelled  to  conclude  that  the  heliotropism  of  such  plants 
is  different  from  that  of  animals,  since  it  would  seem 
preposterous  to  assume  that  swarmspores  of  plants  should 
go  to  the  light  because  they  have  sensations  of  color.  He, 
therefore,  assumes  that  plants  are  heliotropic — in  the 
sense  of  the  mechanistic  theory — but  that  positively  helio- 
tropic animals  go  to  the  light  on  account  of  their  love  for 
brightness  which  is  exactly  the  old  viewpoint  of  Graber. 
It  can  be  shown,  however,  that  the  difference  in  the  helio- 
tropism of  animals  and  plants,  which  Hess  assumes,  is 
contrary  to  the  facts,  since  there  are  heliotropic  animals 
for  which  the  blue  rays  are  the  most  efficient,  as  for  most 
plants ;  and  there  are  green  algae  for  which  the  yellowish- 
green  rays  are  most  efficient,  as  for  animals. 

For  many  if  not  most  plants  the  blue  rays  are  the  most 
efficient  for  inducing  heliotropic  curvatures.  Blaauw47 
proved  this  in  the  following  way:  He  exposed  a  row  of 


WAVE  LENGTH  105 

seedlings  of  Av ena  to  a  carbon  arc  spectrum  for  a  certain 
time.  The  seedlings  were  then  placed  in  the  dark  and 
after  the  proper  time  it  was  ascertained  which  part  of  the 
spectrum  had  induced  heliotropic  curvatures.  By  vary- 
ing the  duration  of  time  of  exposure  to  the  spectrum  it 
was  found  that  with  a  minimal  time  of  exposure  only 
certain  blue  rays,  namely,  those  of  a  wave  length  of  478  w 
caused  heliotropic  bending,  while  with  longer  exposure 
longer  waves  also  became  efficient.  In  this  way  the  mini- 
mum duration  of  exposure  for  various  parts  of  the  spec- 
trum was  ascertained.  Table  IX  gives  his  results. 

TABLE  IX 


Duration  of  illumination,  in  seconds 

Location  of  threshold  in  the  spectrum, 
in  micra 

6.300 

534  [L[i 

1,200 

510  WA 

120 

499^ 

15 

491  w 

5 

487  w* 

4 

478  (jm 

3 

4 

466  WJL 

6 

448ixtx 

The  red  and  yellow  parts  of  the  spectrum  were  ineffec- 
tive for  the  intensity  and  time  limits  used  and  the  optimum 
of  efficiency  was  in  the  blue,  in  the  region  between  466 
and  478  w. 

A  shorter  series  of  experiments  was  made  on  the  fruit 
bearers  of  Phycomyces  f  with  the  following  results : 

44  to  47  per  cent,  of  the  Phycomyces  showed  heliotropic 
curvatures 

after  192  seconds  of  illumination  at  615  fip 
after  192  seconds  of  illumination  at  550  up 
after  16  seconds  of  illumination  at  495  pp 
after  32  seconds  of  illumination  at  450  ^ 
after  64  seconds  of  illumination  at  420  /,/, 


106  TROPISMS 

The  number  of  experiments  was  limited  but  they  indi- 
cate an  optimum  between  495  and  450  ^,  in  this  respect 
agreeing  with  the  results  on  Avena. 

The  fact  then  exists  that  for  the  heliotropic  reactions 
of  certain  plants  the  blue  rays  are  most  efficient,  while 
for  the  heliotropic  reactions  of  a  number  of  animals  the 
yellowish-green  rays  are  most  efficient.  But  this  state- 
ment cannot  be  generalized. 

Loeb  and  Wasteneys  determined  the  most  efficient 
wave  length  of  light  for  various  lower  organisms  with 
the  result  that  there  are  heliotropic  animals  for  which 
the  blue  rays  are  as  efficient  as  they  are  for  plants ;  and 
that  for  different  unicellular  green  organisms  the  opti- 
mum lies  in  different  parts  of  the  spectrum.  They  found, 
by  a  method  similar  to  that  used  by  Blaauw,  that  for  the 
heliotropic  curvature  of  the  animal  Eudendrium  the  most 
efficient  part  of  the  spectrum  lies  in  the  blue  A  =  approxi- 
mately 473  /u/x.811  The  same  was  found  by  them  for  the 
larvae  of  the  marine  worm  Arenicola. 

On  the  other  hand,  on  investigation  of  two  closely 
related  forms  of  green  flagellates,  Euglena  and  Chlamy- 
domonas, it  was  found311  that  they  behave  differently. 
For  Euglena  viridis 'the  blue  rays  A  —  470  to  480  w  are 
especially  efficient,  while  for  Chlamydomonas  pisiformis 
the  most  efficient  part  was  in  the  region  of  A  =  534  ^, 
in  the  yellowish-green.b  For  another  green  algae,  Pan- 
do  rina,  Loeb  and  Maxwell  had  already  found  the  greatest 
efficiency  in  the  greenish-yellow. 

bThis  would  lead  us,  on  the  basis  of  the  reasoning  of  Hess,  to  the 
conclusion  that  the  unicellular  plant  Chlamydomonas  has  sensations  of 
brightness,  suffers  from  total  color  blindness  (although  it  has  no  eyes),  that 
it  is  not  heliotropic,  and  that  it  is  an  animal ;  while  its  unicellular  cousin, 
Euglena,  has  a  highly  developed  color  sense,  has  no  sensations  of  brightness, 
is  heliotropic,  and  is  a  plant. 


WAVE  LENGTH 


107 


The  method  used  for  these  experiments  by  Loeb  and 
Wasteneys  is  as  follows : 

A  carbon  arc  spectrum,  about  from  18  to  23  cm.  wide, 
was  thrown  on  a  black  screen  88  (see  Fig.  34)  with  two 
slits  a  and  b  in  the  two  different  parts  of  the  spectrum 
which  were  to  be  compared  in  regard  to  their  heliotropic 


FIG.  34. — Method  of  determining  the  relative  heliotropic  efficiency  of  two  different  parts 
of  the  spectrum.     (After  Loeb  and  Wasteneys.) 

efficiency.  The  two  beams  of  light  passing  through  the 
slits  are  reflected  by  the  two  mirrors  M  and  M^  into  the 
square  glass  trough  cdfe  in  such  a  way  as  to  strike  the 
same  region  g  of  the  back  wall  of  the  trough.  The  glass 
trough  is  surrounded  by  black  paper  except  at  R  and  R19 
where  the  two  beams  of  light  enter  from  the  mirrors.  Be- 
fore the  experiment  begins,  all  the  organisms  are  collected 
in  the  spot  g  with  the  aid  of  an  incandescent  lamp.  As 


108  TEOPISMS 

soon  as  the  spectrum  is  turned  on,  these  organisms  are 
simultaneously  exposed  to  two  different  beams  of  light 
which  come  from  the  two  mirrors  M  and  M^.  When  one 
type  of  light,  e.g.,  that  from  M,  is  much  more  efficient  than 
the  other  coming  from  M19  practically  all  the  organisms 
are  oriented  by  the  light  from  M  and  move  toward  this 
mirror,  collecting  in  the  region  R.  When  the  relative  effi- 
ciency of  the  two  types  of  light  is  almost  equal,  the  organ- 
isms move  in  almost  equal  numbers  to  R  and  R^.  By 
using  as  a  standard  of  comparison  the  same  region  of 
the  spectrum  and  successively  altering  the  position  of 
the  other  slit  in  the  spectrum,  we  were  able  to  ascertain 
with  accuracy  the  relative  efficiency  of  the  different  parts 
of  the  spectrum  for  the  two  forms  of  organisms.  When 
the  two  parts  of  the  spectrum  which  are  to  be  compared 
are  very  close  to  each  other,  it  is  necessary  to  deflect 
the  beams  with  the  aid  of  deflecting  prisms,  before  they 
reach  the  two  mirrors.311 

Experiments  on  the  newly  hatched  larvae  of  Arenicola, 
a  marine  worm,  showed  that  the  most  efficient  part  of  the 
spectrum  was  in  the  bluish-green  of  about  A  =  495  /«/*, 
while  for  the  larvae  of  Balanus  eburneus  the  most  efficient 
part  of  the  spectrum  was  found  by  Loeb  and  Maxwell, 
by  Hess,  and  by  Loeb  and  Wasteneys  in  the  region  of 
yellow  and  yellowish-green.311 

Mast 348  made  similar  experiments  on  these  organisms 
with  a  method  in  which  the  organisms  were  exposed  to  two 
beams  of  light  of  different  wave  length  crossing  each  other 
at  right  angles.  One  light  was  kept  constant  while  the 
other  was  made  intermittent  by  a  disk  with  a  sector  cut 
out  rotating  in  front  of  the  light.  The  size  of  the  sector 
was  varied  until  the  organisms  moved  at  an  angle  of  45° 
to  the  two  beams.  When  this  happened  the  heliotropic 


WAVE  LENGTH 


a 

Q 

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p 

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Cn  Cn  Cn  Cn  Cn  Cn  rfi. 
Cn  rfi.  CO  tO  i—  '  O  CO 
rfi.  rfi.  rf>.  rfi.  CO  CO  CO 

rfi.  rf^.  rfi.  rfi.  rfi.  rfi.  rfi. 

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bo  co  os  M  bs  to 

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Positive 

2. 

1 

s 

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IS>  1—  k  >—  k  i—  k 

O  CO  OS  tO  <l 

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lit 

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-i 

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CO  OS  Cn  CO  CO 
CO^JOO  •<!  rfi. 

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^c1 

if 

00  Cn  O  ^a 

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i-*O  00  OS 

»—  k 
rf^ 

2S 

j 

Chlamy- 
domonas 

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h-k  t-k  CO  I—  '  O  00  CO 

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1 

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tO  OS  Cn  i—  '  -<i  i—  '  en 
Cn  -vj  i—  *  O  00  Cn  t—  k 

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9 

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k—  k     t-^    t—l 

OS  CO  O  M  Cn 

1 

If 

tO  O  Cn  O  O       Cn  O  O 

ggS^Sgg 

S3S88 

m 

II 

f 


110  TEOPISMS 

efficiency  of  the  two  beams  was  considered  equal.  Mast's 
results,  which  are  given  in  Table  X,  agree  with  those  of 
Loeb  and  Wasteneys. 

The  error  which  Hess  makes  is  of  epistemological  in- 
terest inasmuch  as  it  shows  the  danger  of  false  analogy. 
The  real  analogy  for  heliotropic  reactions  are  forced 
movements  and  other  tropisms,  e.g.,  galvanotropism  or 
geotropism.  Since  forced  movements  (e.g.,  in  Meniere's 
disease)  and  galvanotropic  reactions  caused  by  a  constant 
current  through  our  head  are  not  determined  or  accom- 
panied by  special  sensations,  the  same  may  be  true  in 
regard  to  heliotropic  reactions.  This  is  not  an  idle 
assumption,  since  we  know  that  the  contraction  of  the 
iris  of  our  eye  under  the  influence  of  light  is  not  accom- 
panied by  any  sensation  of  brightness  or  color  and  such 
contractions  occur  also  under  the  influence  of  light  when 
the  iris  is  excised.  Hess  ignores  not  only  this  analogy, 
but  the  whole  existence  of  forced  movements  and  of  other 
tropisms,  and  he  uses  the  color  and  light  sensations  of 
human  beings,  who  are  not  heliotropic,  to  explain  helio- 
tropism  in  animals  about  whose  sensations  we  know  noth- 
ing. He  fails  to  see  that  by  this  false  analogy  he  dodges 
the  real  problem  of  heliotropism,  namely,  why  the  tension 
of  symmetrical  muscles  changes  upon  one-sided  illumina- 
tion of  an  animal.  For  the  explanation  of  this  problem, 
we  find  assistance  in  the  field  of  forced  movements  and  of 
galvanotropism  and  of  geotropism,  but  not  in  the  behavior 
of  totally  color  blind  human  individuals  who  show  no 
trace  of  heliotropism. 

The  adoption  of  the  false  analogy  between  visual  sen- 
sations and  heliotropism  makes  it  impossible  for  Hess 
to  admit  that  bees  should  be  heliotropic  and  at  the  same 
time  be  able  to  discriminate  between  blue  and  gray ;  while 


WAVE  LENGTH  111 

if  we  take  cognizance  of  the  analogy  between  heliotropism 
and  the  other  tropisms  we  realize  that  the  heliotropism  of 
the  bees  and  their  reactions  to  blue  are  separate  and 
independent  phenomena,  which  need  not  be  mutually  ex- 
clusive and  which  in  all  probability  depend  upon  different 
parts  of  the  brain.  When  in  certain  cases  the  relative 
heliotropic  efficiency  of  the  various  parts  of  the  spectrum 
is  identical  with  the  curve  for  its  apparent  relative  bright- 
ness to  a  totally  color  blind  person,  we  may  conclude 
that  the  photosensitive  substances  responsible  for  the  two 
groups  of  phenomena  behave  similarly  or  may  even  be 
identical,  but  not  that  the  sensations  of  brightness  of  the 
color  blind  and  the  heliotropic  reactions  of  insects  are' 
identical  or  analogous  phenomena. 

Many  mutants  of  Drosophila  differ  in  regard  to  the 
pigments  of  the  eye.  It  was  natural  to  raise  the  question 
whether  or  not  such  hereditary  variations  of  pigmentation 
of  the  eye  influence  the  reaction  of  the  flies  to  monochro- 
matic light.  McEwen  investigated  this  possibility  with 
the  following  result :  ' '  Colored  lights  which  may  be  con- 
veniently described  as  violet,  green  and  red,  are  effective 
in  the  order  named  upon  the  insects  whose  eye  color  is 
lighter  than  the  red  eye  of  the  wild  fly.  In  the  case  of 
wild  flies,  and  flies  whose  eyes  are  of  a  still  darker 
shade  called  sepia,  red  is  more  effective  than  green" 
(McEwen549). 


CHAPTER  XII 
CHANGE  IN  THE  SENSE  OF  HELIOTROPISM 

WE  have  stated  that  while  in  a  positively  heliotropic 
animal  a  one-sided  illumination  increases  the  tension  of 
the  muscles  which  turn  the  animal  toward  the  source  of 
light,  in  the  negatively  heliotropic  animal  the  one-sided 
illumination  must  result  in  the  opposite  effect,  namely,  in 
a  diminution  of  tension  in  the  same  muscles.  As  a  conse- 
quence, the  negatively  heliotropic  animal  can  turn  more 
easily  away  from  the  light  than  toward  the  light. 

Groom  and  Loeb  183  noticed  that  the  larvae  of  the  bar- 
nacle upon  hatching  go  directly  to  the  light  and  gather 
at  the  light  side  of  a  dish,  but  that  sooner  or  later  their 
positive  heliotropism  may  give  way  to  an  equally  pro- 
nounced negative  heliotropism.  The  stronger  the  light  the 
more  rapidly  the  larvae  are  transformed  into  negatively 
heliotropic  organisms.  Later  the  reversibility  of  the  sense 
of  heliotropism  was  observed  and  studied  in  a  number 
of  organisms.291  In  a  summary  of  the  subject  30°  (p.  470) 
the  writer  pointed  out  that  this  reversion  was  due  either 
to  a  modification  of  photochemical  processes  or  to  an 
effect  upon  the  nervous  system.  That  an  influence  on  the 
nervous  system  can  indeecl  bring  about  a  change  in  the 
sign  of  the  reaction  is  very  strikingly  demonstrated  in  the 
following  observation  of  A.  E.  Moore  on  starfish.525 

Ordinarily,  when  a  starfish  which  is  moving  in  an  aquarium  is 
touched,  it  stops  immediately  and  clings  tenaciously  to  the  surface  of 
the  vessel  with  its  tube  feet,  so  that  it  is  impossible  to  remove  the  animal 
without  injury  to  the  tube  feet.  This  normal  response  to  sudden  contact 
can  be  completely  reversed  by  the  administration  of  strychnine,  so  that 
when  touched  the  animal  loosens  its  hold  on  the  bottom  completely. 
112 


HELIOTBOPIC  TEANSFOEMATION          113 

The  starfish  poisoned  with  strychnine  upon  sudden 
touch  withdraws  all  the  tube  feet,  so  that  it  can  be  moved 
about  like  an  inert  object.  For  this  purpose  1  or  2  c.c.  o±' 
a  0.5  per  cent,  solution  of  strychnine  sulfate  were  injected 
into  a  starfish  of  medium  size. 

If  the  stretching  out  of  the  tube  feet  is  due  to  an  in- 
crease in  the  tone  of  the  ring  muscles  (and  a  decrease 
in  the  tension  of  the  longitudinal  muscles)  the  drawing 
in  is  due  to  an  increase  in  the  tone  of  the  longitudinal 
muscles  of  the  tube  feet.  We  therefore  see  that  the  same 
"  stimulus, "  namely,  a  sudden  touch,  which  causes  one 
set  of  muscles  to  contract  in  a  normal  animal  causes  the 
antagonists  of  these  muscles  to  contract  in  an  animal 
poisoned  with  strychnine.  We  shall  see  that  a  number  of 
cases  of  reversal  of  heliotropism  may  well  find  their 
explanation  on  this  basis.  On  the  other  hand,  the  phe- 
nomena of  solarization  known  in  photography  indicate 
that  the  sign  of  heliotropic  response  may  also  be  changed 
by  an  excessive  action  of  light  on  the  photochemical  sub- 
stance. This  effect,  of  course,  may  in  the  last  analysis 
also  result  in  an  influence  upon  the  central  nervous  system, 
such  as  that  brought  about  by  strychnine  in  Moore 's  ex- 
periment. We  will  now  consider  some  cases  more  in 
detail. 

The  writer  found296  that  certain  fresh  water  crusta- 
ceans, namely  Californian  species  of  Daphnia,  copepods, 
and  Gammarus  when  indifferent  to  light  can  be  made 
intensely  positively  heliotropic  by  adding  some  acid  to  the 
fresh  water,  especially  the  weak  acid  C02.  When  car- 
bonated water  (or  beer)  to  the  extent  of  about  5  or  10  c.c. 
is  slowly  and  carefully  added  to  50  c.c.  of  fresh  water 
containing  these  Daphnia,,  the  animals  will  become  in- 
tensely positive  and  will  collect  in  a  dense  cluster  on  the 

8 


114  TKOPISMS 

window  side  of  the  dish.  Stronger  acids  act  in  the  same 
way  but  the  animals  are  liable  to  die  quickly.  Esters, 
e.g.,  ethylacetate,  act  also  like  acids  and  the  addition  of 
1  c.c.  of  a  grammolecular  solution  of  ethylacetate  to  50  c.c. 
fresh  water  also  makes  all  the  organisms  positively  helio- 
tropic.  Alcohols  act  in  the  same  way.  In  the  case  of 
Gammarus  the  positive  heliotropism  lasts  only  a  few 
seconds,  while  in  DapJmia  it  lasts  from  10  to  50  minutes 
and  can  be  renewed  by  the  further  careful  addition  of 
some  C02.  The  following  table  gives  the  minimal  con- 
centration of  various  acids  and  alcohols  for  the  production 
of  positive  heliotropism  in  certain  California  species  of 
fresh  water  copepods,  and  Daphnia: 

For  Copepods  For  Daphnia 

Formic  acid   0.006  N 

Acetic  acid    0.006  N 

Propionic  acid   0.005,  N 

Butyric   acid    0.004  N 

Valerianic  acid    0.004  N 

Capronic  acid   0.002  N  0.6  N 

Ethyl  alcohol    0.19    N  0.2  N 

Propyl  alcohol   0.054  N  0.05  to  0.1  N 

Normal  butyl  alcohol 0.019  N 

Isobutyl  alcohol   0.04  N 

Amyl  alcohol    0.011  N 

As  far  as  alcohols  are  concerned  each  higher  alcohol  is 
about  three  times  as  efficient  as  the  previous  one,  with 
the  exception  of  amyl  alcohol.  This  order  of  relative 
efficiency  is  also  characteristic  for  the  surface  tension 
effects  of  these  alcohols.299 

It  was  of  importance  to  find  means  of  making  these 
organisms  negatively  heliotropic.  Moore368  found  that 
caffein  makes  the  heliotropically  indifferent  fresh  water 
crustacean  Diaptomus  intensely  negatively  heliotropic. 
It  required  the  addition  of  1.2  c.c.  of  a  1  per  cent,  solution 
of  caffein  to  50  c.c.  of  water  to  bring  about  this  intense 


HELIOTEOP1C  TRANSFORMATION          115 

negativation.  In  two  minutes  all  the  animals  are  col- 
lected in  a  dense  cluster  on  the  negative  side  which  lasts 
for  about  35  minutes.  A  weak  negative  collection  could 
also  be  obtained  by  adding  0.1  c.c.  of  a  0.5  per  cent,  solu- 
tion of  strychnine  nitrate.  Moore  found  that  if  the  Diap- 
tomus  were  first  made  positively  phototropic  by  the  addi- 
tion of  alcohol  or  acids,  it  was  impossible  to  alter  their 
response  by  the  action  of  caffein,  strychnine,  or  atr opine. 
On  the  other  hand,  animals  which  had  formed  a  negative 
collection  under  the  influence  of  caff  ein  if  treated  with  car- 
bonated water  at  once  changed  their  response  and  swim- 
ming to  the  light  side  of  the  dish  formed  a  positive 
gathering. 

What  causes  these  effects?  The  fact  that  alcohols 
make  the  organisms  positively  heliotropic  suggested  the 
possibility  of  a  " narcotic "  effect;  the  writer  found,  how- 
ever, that  narcosis  requires  a  concentration  of  alcohols 
three  times  as  high  as  the  one  required  to  produce  positive 
heliotropism.  He  tried  the  effect  of  temperature  on 
the  reversal  of  the  sign  of  heliotropism  in  Daphnia  and 
found  that  lowering  of  the  temperature  enhanced  the 
effect  of  acids  in  making  the  animals  positive.296 

The  writer  had  found  previously  that  in  marine  crus- 
taceans and  in  larvae  of  a  marine  annelid,  Polygordius,  the 
sense  of  heliotropism  can  be  reversed  by  changes  of  tem- 
perature as  well  as  by  changes  in  the  osmotic  pressure 
of  the  sea  water.291  Increase  in  the  osmotic  pressure  of 
sea  water  (by  adding  about  1  gm.  of  NaCl  or  its  osmotic 
equivalent  of  other  substances  to  100  c.c.  of  sea  water) 
made  the  negative  animals  positively  heliotropic,  and 
lowering  of  the  concentration  by  adding  30  to  60  c.c.  dis- 
tilled water  to  100  c.c.  sea  water  made  positive  organisms 
negative.  Negative  larvae  of  Polygordius  or  negative 


116  TEOPISMS 

marine  copepods  could  be  made  positive  by  lowering  the 
temperature,  and  positive  larvae  could  be  made  negative 
by  slowly  raising  the  temperature.  Since  in  the  latter 
case  the  animals  suffered  from  the  high  temperature  the 
results  were  not  so  striking  as  in  the  case  of  the  positivat- 
ing  effect  of  lowering  the  temperature.  The  same  effect 
of  the  concentration  of  sea  water  and  of  temperatures 
was  observed  by  Ewald  for  the  larvae  of  Balanus  perfor- 
alus.  He  found,  moreover,  the  interesting  fact  that  a 
change  of  the  ratio  ~~  in  the  sea  water  affected  the  sign 
of  heliotropism  of  barnacle  larvae.  An  increase  of  Na 
made  them  more  positive,  an  increase  in  Mg  more 
negative.144 

The  larvae  of  Porthesia  are  strongly  positively  helio- 
tropic  before  they  have  eaten,  while  they  lose  their  helio- 
tropism almost  completely  after  they  have  eaten.287  The 
writer  observed  that  male  and  female  winged  ants  are 
strongly  positively  heliotropic  but  as  soon  as  they  lose 
their  wings  their  heliotropism  ceases.287  McEwen 549 
has  found  that  when  Drosophila  is  deprived  of  its  wings 
its  heliotropism  ceases. 

Holmes  found  that  terrestrial  amphipods  are  posi- 
tively, while  the  aquatic  amphipods  are  negatively  helio- 
tropic. By  putting  a  terrestrial  amphipod  into  water  it 
became  negatively  heliotropic.225 

That  a  reversal  in  the  sense  of  heliotropism  may  be 
due  to  a  nervous  effect  is  suggested  by  an  observation 
by  Miss  Towle 485  that  a  certain  ostracod,  Cypridopsis, 
can  be  made  positively  heliotropic  by  mechanical  shock, 
and  the  writer  noticed  that  indifferent  fresh  water  Gam- 
marus  can  be  made  negatively  heliotropic  by  shaking 
them.  In  both  cases  the  heliotropism  lasts  only  a  short 
time. 


HELIOTEOPIC  TEANSFOEMATION          117 

The  attempt  to  explain  all  these  reversals  on  the 
assumption  of  a  change  in  the  central  nervous  system 
meets  with  the  difficulty  that  such  reversals  occur  also  in 
unicellular  organisms  which  have  no  central  nervous 
system.  Thus  the  writer  observed  that  Volvox,  which 
occurred  in  the  same  ponds  in  California  from  where 
Daphnia  came,  could  also  be  made  positive  by  C02.296 
In  swarmspores  of  algae  reversals  of  heliotropism  are  a 
common  phenomenon.  While  these  unicellular  organisms 
have  no  central  nervous  system  they  may  have  synapses 
such  as  exist  between  different  neura  of  metazoa.  The 
writer  is  not  sufficiently  familiar  with  the  behavior  of 
synapses  in  higher  animals  to  suggest  that  this  condition 
is  responsible  for  the  changes  in  the  sense  of  heliotropism. 

We  may  finally  discuss  briefly  a  possible  solarization 
effect.  The  writer  found  that  it  is  possible  to  make  ani- 
mals generally  negatively  heliotropic  with  the  aid  of 
ultraviolet  light.296  If  once  rendered  negative  such  ani- 
mals will  be  negative  not  only  to  ultraviolet  rays  but  also 
to  the  light  of  an  incandescent  lamp.  A.  E.  Moore366 
found  that  the  ultraviolet  rays  having  such  an  effect  have 
a  wave  length  shorter  than  3341  A.U.  Oltmanns  had  ob- 
served that  Phycomyces  is  positively  heliotropic  in  weak 
light,  indifferent  in  somewhat  stronger  light,  and  nega- 
tively heliotropic  in  still  stronger  light.  Blaauw  found 
that  when  the  illumination  was  strong  the  seedlings  of 
Avena  became  negatively  heliotropic.47  He  suggests  the 
analogy  with  solarization  effects  in  photography.  The 
discovery  of  photodynamic  effects  by  v.  Tappeiner477 
adds  to  the  possibilities  which  should  be  considered  in 
this  connection. 

While  Drosophila  is  usually  positively  heliotropic, 
McEwen  has  recently  described  a  mutant  of  this  species 


118  TEOPISMS 

which  is  not  heliotropic.  This  lack  of  heliotropic  response 
is  linked  with  a  peculiar  color — "tan" — by  which  the 
mutant  is  characterized.  The  character  "tan"  is  sex 
linked.  The  daughters  inherit  the  factor  for  the  character 
from  their  fathers  but  do  not  show  the  character,  while 
the  sons  inherit  the  factor  from  their  mothers  and  do 
show  the  character.  The  lack  of  heliotropic  reaction  in 
this  mutant  is  apparently  not  due  to  any  structural 
defect  in  the  eye  (McEwen549). 

Keeping  successive  generations  of  flies  in  the  dark 
does  not  influence  their  heliotropism.  F.  Payne 550' 551 
raised  sixty-nine  successive  generations  of  Drosophila 
in  the  dark,  but  the  reaction  of  the  insects  to  light  (as 
well  as  their  eyes)  remained  entirely  normal. 


CHAPTER  XIII 

GEOTEOPISM 

1.  When  the  stem  of  certain  plants  is  placed  in  a 
horizontal  position,  the  apex  grows  vertically  upward  and 
the  root  downward.  The  downward  growth  of  the  root 
is  called  positive,  the  upward  growth  of  the  apex  nega- 
tive geotropism.  The  writer  has  observed  a  similar  phe- 
nomenon in  a  hydroid,  Antennularia  antennina 294'  30° 
and  his  observations  were  confirmed  by  Miss  Stevens.553 
Animals  as  well  as  plants,  therefore,  show  the  phenomenon 
of  geotropism. 

These  phenomena  have  given  rise  to  a  strange  dis- 
cussion, namely :  What  constitutes  the  '  '  stimulus ' '  in  the 
case  of  geotropism?  When  a  galvanic  current  is  sent 
through  a  motor  nerve  the  muscle  answers  with  a  con- 
traction only  when  the  current  is  made  or  broken,  but 
not  while  a  constant  current  is  flowing  through  the  nerve. 
The  older  physiologists  were  not  able  to  form  a  mental 
picture  of  what  happened  in  this  case,  and  they  cut  the 
knot  by  invoking  a  verbalism,  namely  by  calling  the  mak- 
ing or  breaking  of  a  current  a  ' l  stimulus.  ' '  This  perhaps 
innocent  verbalism  then  led  to  the  less  harmless  dogma 
that  only  a  rapid  change  could  act  as  a  "  stimulus. ' '  Thus 
Jennings 253  and  Mast 346  took  it  for  granted  that  phe-^ 
nomena __pf _orientatiQii_  by Jight  could  only  be  produced 
by  rapid  changes  in  the  intensity  of  light  and  not  by 
constant  illumination,  since  they  had  the  a  priori  convic- 
tion  that  only  a  rapid  change  in  the  intensity  of  a  gal- 
vanic current  or  of  light  is  a  "  stimulus. "  The  same  diffi- 

119 


120  TEOPISMS 

culty  arose  in  regard  to  the  action  of  gravity  upon  orien- 
tation, since  it  was  contrary  to  the  definition  of  a  i  l  stimu- 
lus ' '  that  the  mere  permanent  lying  in  a  horizontal  posi- 
tion should  cause  the  apex  of  a  stem  to  bend  upward. 

All  these  difficulties  disappear  if  we  take  the  law 
of  chemical  mass  action  into  consideration.  Light  acts 
not  as  a  "stimulus"  but  acts  by  increasing  the  mass 
of  certain  chemical  compounds,  and  it  is  the  mass  of 
these  products  which  is  responsible  for  the  effect  of  light. 
Now,  mass  action  is  not  proportional  to  the  rapidity 
of  the  change  of  acting  masses  but  to  the  acting  mass 
itself.  When  two  sides  of  an  organism  are  struck  by 
light  of  different  intensity  the  quantity  of  photochemical 
products  on  both  sides  becomes  unequal.  In  galvano- 
tropism  the  galvanic  current  alters  the  distribution  of  the 
mass  of  certain  ions  along  the  nerve  elements. 

It  can  be  shown  that  gravitation  acts  by  influencing 
|  the  distribution  of  chemical  substances  in  an  organism. 
'!  When  the  stem  of  a  plant  is  put  into  a  horizontal  position 
!  certain  chemical  substances  gather  in  greater  concen- 
tration on  the  lower  side  of  the  stem ;  and  this  causes  a 
difference  in  the  velocity  of  chemical  reactions  between 
<  the  lower  and  the  upper  side.    As  a  result  of  this  we 
•notice  the  bending.     In  the  normal  upright  position  of 
the  plant  the  same  substances  were  distributed  equally 
about  the  axis  of  symmetry. 

The  following  facts  may  be  offered  as  a  proof  for  this 
statement.526  When  we  put  a  piece  of  the  stem  of  Bryo- 
phyllum,  calycinum  in  a  horizontal  position  it  soon  bends 
and  gradually  assumes  the  form  of  a  U  with  the  concave 
side  above  (Fig.  35).  This  bending  is  due  to  the  fact  that 
the  cortex  on  the  under  side  of  the  stem  grows  in  length 
while  the  cortex  on  the  upper  side  remains  unaltered 


FIG.   35. — Geotropic  curvature  of  stems  of  Bryophyllum  calycinum.     These  stems  were 
originally  straight  and  suspended  in  a  horizontal  position.     In  about  ten  days  they  bent, 
becoming  concave  on  the  upper  side.  -  The  black  rings,  made  with  india  ink,  which  were 
originally  parallel,  remain  unaltered  on  the  upper  side  of  the  stems,  while  their  distance  \  V« 
increases  on  the  lower  side,  indicating  that  the  curvature  is  due  to  an  increase  in  growth  )  " 
on  the  lower  side  (of  the  cortex)  of  the  stem. 


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GEOTROPISM  121 

(Loeb542).  This  can  be  demonstrated  if  we  mark  the 
cortex  in  definite  intervals  with  india  ink  at  the  begin- 
ning of  the  experiment  (Fig.  35).  After  some  time  the 
distance  between  these  marks  will  increase  in  a  certain 
region  of  the  under  side,  while  it  remains  constant  on  the 
upper  side,  and  this  difference  causes  the  bending.  This 
positive  increase  in  length  of  the  under  side  can  only 
happen  through  growth,  and  this  growth  of  the  cortex 
on  the  lower  side  of  the  stem  takes  place  at  the  expense 
of  material  furnished  constantly  by  the  leaves  which  send 
it  in  the  direction  toward  the  basal  end  of  the  stem.  When 
we  compare  the  rate  of  geotropic  bending  of  horizontal 
stems  without  leaves  and  with  one  or  two  leaves  at  the 
apex,  we  find  that  the  bending  in  the  latter  is  much  more 
rapid  (Fig.  36),,  owing  to  the  greater  mass  of  material 
supplied  for  the  growth  of  the  cortex,  and  the  same  is  true, 
if  we  compare  the  rate  of  curvature  of  stems  having  a 
whole  apical  leaf  attached  with  that  of  stems  having  an 
apical  leaf  whose  mass  has  been  reduced  by  cutting  off 
parts  of  the  leaf  (Figs.  37  and  38).  The  writer  has  shown 
in  other  experiments  that  under  equal  conditions  leaves 
produce  material  fit  for  growth  in  proportion  to  their 
mass.  It  is,  therefore,  a  safe  inference  that  the  influence 
of  the  mass  of  an  apical  leaf  upon  the  rate  of  geotropic 
bending  is  due  to  the  mass  of  material  it  sends  into  the 
stem.  This  material  has  obviously  a  tendency  to  behave 
like  a  liquid — which  it  probably  is — and  to  sink  to  the 
lower  level.  It  is,  therefore,  useless  to  look  for  a  "  gravi- 
tational stimulus.  »526,544 

What  has  been  demonstrated  in  this  case  explains 
probably  also  why  the  apex  of  many  plants  when  put  into  a 
horizontal  position  grows  upward,  and  why  certain  roots 
under  similar  conditions  grow  downward.  It  disposes 


122  TBOPISMS 


also  in  all  probability  of  the  suggestion  that  the  apex 
of  a  positively  geotropic  root  has  "  brain  f unctions. "  It 
is  chemical  mass  action  and  not  "brain  functions"  which 
are  needed  to  produce  the  changes  in  growth  underlying 
geotropic  curvature. 

2.  As  long  as  animals  are  in  such  a  position  that  their 
plane  of  symmetry  goes  through  the  center  of  the  earth, 
the  position  of  their  eyes  and  limbs  is  symmetrical  in 
regard  to  their  plane  of  symmetry.  If,  however,  we  incline 
the  animal  we  can  bring  about  forced  movements  and 
forced  changes  of  position  of  the  same  nature  a&  those 
i  caused  by  injury  of  one  side  of  certain  parts  of  the  brain. 
Thus  we  have  seen  that  if  we  cut  the  left  side  of  the 
medulla  oblongata  in  a  shark,  its  two  eyes  are  no  longer 
in  a  symmetrical  position  but  the  left  eye  looks  down  and 
the  right  eye  up,  when  the  shark  is  kept  in  a  normal  posi- 
tion. The  same  change  can  be  brought  about  in  a  normal 
shark  by  the  influence  of  gravitation.  When  the  shark  is 
kept  in  a  position  with  its  right  side  inclined  downward, 
the  right  eye  is  turned  upward,  the  left  eye  downward. 
This  has  nothing  to  do  with  light  or  vision,  since  it  occurs 
in  the  dark  just  as  well  as  in  an  illuminated  room.  The 
abnormal  position  of  the  eyes  lasts  as  long  as  the  animal 
is  kept  in  this  abnormal  position.  The  experiment  shows 
that  if  the  plane  of  symmetry  is  no  longer  vertical,  forced 
positions  of  the  eyes  can  be  produced  of  the  same  nature 
as  those  produced  by  one-sided  injury  of  certain  parts  of 
the  brain. 

Just  as  in  the  case  of  one-sided  injury  to  the  medulla 
oblongata(^the  changes  in  the  position  of  the  eyes  are 
accompanied  by  changes  in  the  position  of  the  pectoral 
fins,  so  also  when  we  put  a  normal  shark  with  one  side 
downward  or  half  downward.289  If  the  right  side  of  such 


GEOTROPISM  123 

a  fish  is  down  the  right  pectoral  fin  is  turned  more  ven- 
trally,  the  left  fin  is  turned  more  dorsally.  This  means, 
the  tension  of  the  muscles  causing  the  right  fin  to  press 
down  and  the  left  fin  to  press  up  is  increased.  This  is 
the  mechanism  by  which  the  normal  "  equilibrium M  or 
more  correctly  the  normal  geotropic  orientation  of  the 
animal  is  maintained.  If  the  animal  should  accidentally 
roll  to  one  side  in  its  normal  movements,  the  tension  of 
the  muscles  of  the  pectoral  fins  would  automatically 
change  in  such  a  way  as  to  restore  the  normal  orientation 
of  the  animal,  whereby  the  plane  of  symmetry  becomes 
vertical  again.  This  "maintenance  of  equilibrium"  is 
therefore  a  case  of  automatic  orientation  by  gravitation 
comparable  to  the  automatic  orientation  by  light.  / 

Geotropic  changes  in  the  position  of  the  eyes  are  not 
confined  to  fishes,320  they  can  be  demonstrated  in  a  rabbit 
and  in  crustaceans  as  well. 

In  vertebrates  the  reactions  leading  to  the  maintenance 
of  equilibrium  are  apparently  produced  in  the  ear,  since 
they  disappear  if  the  acoustic  nerves  are  cut.  Moreover, 
those  parts  of  the  brain  whose  injury  brings  about  such 
changes  in  the  position  of  the  eye  and  the  fins  are  parts 
of  the  receiving  fibers  from  the  acoustic  nerve.290 

It  seems  that  some  change  in  the  pressure  upon  the 
endings  of  the  auditory  nerve  is  responsible  for  the  effects. 
There  are  fine  grains  of  CaCO3 — the  otoliths — in  the  ear 
of  many  species  pressing  on  the  underlying  nerve  end- 
ings. If  we  put  the  median  plane  of  a  fish  at  an  angle 
of  45°  with  the  vertical,  the  otoliths  will  no  longer  press 
down  equally  in  both  ears.  The  idea  first  suggested  by 
Delage  that  it  is  the  pressure  of  the  otoliths  upon  the 
nerve  endings  which  is  responsible  for  these  reactions 
receives  some  support  by  a  well-known  experiment  by 


124  TEOPISMS 

Kreidl.270  A  crustacean,  Palcemon,  loses  its  otoliths  in 
the  process  of  moulting  and  the  animal  curiously  enough 
replaces  them  by  picking  up  small  grains  of  sand  and  put- 
ting them  into  its  ears.  Kreidl  kept  such  crustaceans  in 
jars  free  from  sand  but  containing  line  particles  of  iron 
which  the  crustaceans  after  moulting  put  into  their  ears. 
He  expected  that  a  magnet  would  now  influence  the  ani- 
mals as  powerfully  as  gravitation,  and  this  was  the  case. 
When,  e.g.,  he  brought  a  magnet  from  above  and  the  right 
near  the  animal  the  latter  turned  to  the  left  and  downward. 
The  animal,  therefore,  behaved  as  if  changes  of  pressure 
of  the  otolith  upon  the  nerve  endings  determined  its 
geotropic  orientation. 

The  theory  meets  with  two  difficulties  which,  however, 
are  not  insuperable.  First,  removal  of  all  the  otoliths 
does  not  interfere  with  the  normal  orientation  of  the  ani- 
mal. This  might  find  its  explanation  in  the  fact  that  the 
eyes  act  as  a  substitute.  Delage  had  shown  that  if  the 
otocysts  are  removed  in  crustaceans  or  cephalopods  the 
animals  lose  their  normal  orientation  more  easily  when 
they  swim  about  excitedly  than  do  normal  animals.  In 
order  to  show  the  effects  clearly,  however,  it  was  neces- 
sary to  blind  the  animals.  Animals  which  were  merely 
blinded  but  had  their  otocysts  did  not  show  these  disturb- 
ances of  equilibrium.119 

The  second  difficulty  is  the  fact  that  animals  which 
possess  naturally  no  otoliths  are  yet  able  to  show  such 
geotropic  reactions,  e.g.,  certain  crustaceans  like  Gelasi- 
mus  and  Platyonichus.  We  may  assume  that  the  pres- 
sure of  liquids  on  the  nerve  endings  may  have  a  similar 
effect  as  the  pressure  of  the  otoliths. 

The  next  question  is,  How  does  the  pressure  on  a  nerve 
ending  bring  about  changes  in  the  tension  of  muscles? 


GEOTROPISM  125 

We  suspect  that  this  occurs  through  a  change  in  mass 
action  in  the  nerve  endings,  in  analogy  to  our  experiments 
on  the  influence  of  the  mass  of  the  leaf  on  the  geotropic 
curvature  of  Bry&phyllum,  but  experimental  data  are 
lacking. 

3.  We  observe  phenomena  of  geotropism  in  animals ( 
which  have  no  ears,  but  this  need  not  surprise  us  in  view 
of  the  observations  on  geotropism  in  plants,  and  in  hy- 
droids    (Antennularia  antennina).     The  writer289  had 
found  that  a  holothurian  (Cucumaria  cucumis)  has  a  ten- 
dency to  creep  upward  when  put  on  a  vertical  object  until 
it  reaches  the  highest  level,  where  it  remains.    When  put 
on  a  vertical  plate  of  glass  or  slate,  these  animals  creep 
untiringly  upward  if  only  the  plate  is  turned  180°  around 
a  horizontal  axis  as  soon  as  they  have  reached  the  highest 
point.    It  could  be  shown  that  light  and  oxygen  supply 
have  nothing  to  do  with  the  phenomenon.  ( Jennings 
observed  that  Param&cia  always  gather  at  the  highest 
point  of  a  vertical  tube  and  that  they  assume  this  position 
by  active  ciliary  motion.     Lyon323  assumes  that  the  body 
of  Paramcecia  contains  substances  of  different  density 
whose  location  is  changed  by  changes  in  orientation  of 
the  organism  to  the  center  of  the  earth  and  that  these 
changes  automatically  turn  the  animal  again  so  that  its 
oral  pole  is  directed  upward.     It  will  then  continue  to 
swim  in  this  direction./' 

4.  It  is  known  since  Knight 's  experiments  that  cen- 
trifugal force  can  act  like  gravitation  and  we  must  assume 
that  the  centrifugal  force  leads  to  an  alteration  in  the 
distribution  of  the  sap  or  of  other  substances  in  the  cell. 
This  leads  to  differences  in  the  rate  of  chemical  reactions 
and  may  account  for  the  phenomena  of  orientation  under 
the  influence  of  centrifugal  force. 


126  TEOPISMS 

When  an  animal,  e.g.,  a  shark  or  a  pigeon,  is  rotated  on 
a  turntable,  during  rotation  a  nystagmus  is  observed  in  the 
motions  of  the  eyes  and  sometimes  also  of  the  head.  If 
the  rotation  is  not  too  rapid  the  eyes  move  slowly  in  the 
same  plane  but  in  an  opposite  direction  from  the  rotation 
of  the  turntable,  until  they  form  a  maximum  angle  with 
their  normal  position  in  the  head ;  then  they  rapidly  swing 
back  and  the  whole  phenomenon  is  repeated.  This  phe- 
nomenon is  called  nystagmus.  It  depends  upon  the  nerve 
endings  in  the  semicircular  canals,  but  is  not  dependent 
upon  the  motion  or  pressure  of  the  lymph  in  the 
canals,290*319'320  since  the  cutting  out  of  the  canals  in 
the  shark  or  the  plugging  up  of  the*canals  in  the  pigeon  141 
leaves  the  phenomenon  unaltered.  When  after  some  rota- 
tion the  motion  of  the  turntable  suddenly  stops,  a  nystag- 
mus of  the  eyes  or  head  in  the  same  plane  but  in  the  oppo- 
site direction  as  during  the  rotation  is  observed. 

Maxwell 554  has  shown  that  if  Phrynosoma  is  rotated 
on  a  horizontal  plane  with  constant  velocity  and  the  eyes 
of  the  animal  are  closed,  compensatory  motions  of  the  head 
are  produced  as  soon  as  the  angular  velocity  exceeds  a 
certain  value  which  was  8  seconds  for  a  rotation  through 
an  angle  of  45°. 


CHAPTER  XIV 

FORCED  MOVEMENTS  CAUSED  BY  MOVING 
EETINA  IMAGES :  EHEOTEOPISM : 
ANEMOTEOPISM 

THE  experiments  on  forced  movements  show  that  we 
have  three  groups  of  forced  movements,  (1)  right  to 
and  left  to  right  (circus  movements) ;  (2)  forward  move- 
ment, and  (3)  backward  movement.  The  latter  is  not 
always  possible.  A  fourth  group,  the  rolling  motio 
around  the  longitudinal  axis  may  be  omitted  here  in 
to  simplify  the  discussion. 

The  forced  movements,  called  forth  by  the  galvanic 
current,  supported  the  idea  that  the  nervous  elements 
determining  these  motions  must  have  a  definite  orientation 
and  that  this  orientation  bears  some  simple  relation  to 
the  direction  of  motion  caused  by  their  activity.  The 
experiments  on  the  effect  of  blackening  different  parts 
of  the  eye  indicate  that  the  different  parts  of  the  retinae  of 
positively  heliotropic  insects  are  connected  in  a  simple 
way  with  the  main  centers  of  the  three  types  of  forced 
movements :  namely,  the  left  eye  is  connected  with  the 
brain  center  causing  motions  from  right  to  left  (and  the 
right  eye  with  the  center  for  the  opposite  motion) ;  the 
lower  halves  of  the  retina  with  the  forward  movements, 
the  upper  halves  with  the  backward  movements. 

We  know  through  the  work  of  Ewald  Hering  that  each 
illuminated  element  on  the  human  retina  determines  a 
definite  motion  of  the  two  eyes  which  move  as  if  they  were 
a  single  organ,  and  that  this  motion  is  a  function  of  the 

127 


128  TEOPISMS 

location  of  the  illuminated  element  in  the  retina.  This 
fact  induced  the  writer  to  suggest  in  his  first  publication 
on  tropisms  that  the  act  of  focussing  in  our  vision  was 
simply  a  phenomenon  of  heliotropism.  "The  general 
principle  of  orientation  of  organisms  to  light  is  also  mani- 
fested in  our  act  of  binocular  vision  which  results  auto- 
matically in  such  an  orientation  of  the  two  retinae  that 
the  image  of  the  luminous  point  falls  upon  the  two  foveae 
centrales  of  the  retinae "  (which  are  symmetrical  ele- 
ments). In  other  words,  when  an  object  causes  us  to  turn 
our  eyes  to  it  we  are  dealing  with  a  phenomenon  of  forced 
(heliotropic)  movement.  In  order  to  prove  this  it  is  neces- 
sary to  show  that  a  moving  retina  image  can  produce 
forced  movements  determined  by  the  direction  of  motion 
of  the  luminous  object.  The  difficulties  inherent  in  the 
proof  for  such  a  statement  lie  in  the  general  prejudice 
that  the  motions  of  an  animal  are  directed  to  a  purpose 
and  it  is,  therefore,  necessary  to  devise  experiments  which 
exclude  the  assumption  of  an  interest  on  the  part  of  the 
animal  in  the  motion. 

The  writer  observed  years  ago  that  when  a  fly  is  put 
on  a  rotating  disk  it  rotates  in  the  opposite  direction 
from  the  disk.  When  the  motion  of  the  turntable  ceases 
these  compensatory  motions  of  the  fly  stop  also  and  none 
of  the  after  effects  mentioned  at  the  end  of  the  previous 
chapter  are  noticed.286  This  suggested  that  the  so-called 
compensatory  motions  of  insects  on  the  turntable  have  a 
different  origin  from  that  of  vertebrates.  The  phenom- 
enon was  explained  by  Eadl,  who  proved  that  the  com- 
pensatory motions  of  insects  on  the  turntable  are  pro- 
duced in  the  eye  and  that  they  are  due  to  the  fact  that  the 
eye  tries  automatically  to  fix  the  same  object.447  This 
agrees  with  the  observation  of  Lyon  who  had  already 


RHEOTKOPISM  129 

demonstrated  previously  that  the  compensatory  motions 
of  insects  on  a  turntable  stop  when  their  eyes  are  black- 
ened.319 Such  forced  motions,  due  to  the  influence  of 
the  motion  of  the  retina  image,  can  be  demonstrated  in 
the  Californian  lizard  Phrynosoma  blainvilli,  which  is 
an  ideal  object  for  such  experiments 298  and  in  this  animal 
it  is  possible  to  separate  these  effects  from  the  compensa- 
tory motions  caused  by  centrifugal  force. 

It  was  accidentally  observed  by  the  writer  that  when 
the  lizard  Phrynosoma  is  kept  at  the  window  of  a  moving 
train  with  its  eyes  toward  the  window,  a  nystagmus  of  the 
head  of  the  lizard  ensues,  the  head  moving  slowly  in  a 
direction  opposite  to  that  of  the  moving  train,  as  if  to  keep 
its  eyes  fixed  on  the  objects  outside — telegraph  poles  and 
trees,  etc.  The  head  moves  until  it  is  bent  maximally, 
when  it  is  brought  back  into  its  normal  position  with  a 
quick  jerky  movement,  and  then  follows  again  the  appa- 
rent motion  of  the  objects  outside,  and  so  on.  These  nys- 
tactic  motions  last  for  hours,  in  fact  as  long  as  the  animal 
is  kept  with  its  head  toward  the  window.  As  soon  as  it 
is  turned  around  so  that  it  cannot  see  the  objects  outside, 
the  nystactic  motions  of  the  head  cease.  When  the  animal 
is  put  on  a  turntable  and  rotated  slowly,  vigorous  com- 
pensatory movements  can  also  be  observed  during  rota- 
tion. If,  however,  the  eyes  of  the  lizard  .are  closed  during 
rotation  these  movements  are  considerably  diminished 
though  they  do  not  cease  entirely.  They  are  also  consider- 
ably diminished  when  the  animal  with  its  eyes  op'en  is 
rotated  on  a  turntable  surrounded  by  a  high  gray  cylinder 
of  cardboard  which  excludes  the  possibility  of  images 
of  outside  objects  moving  on  the  retina.  We  can  also 
produce  compensatory  motions  of  the  head  if  the  animal 
9 


130  TEOPISMS 

is  kept  quiet  and  objects  are  moved  in  front  of  it,  the 
eyes  following  the  moving  object. 

It  is  of  interest  to  separate  the  nystagmus  or  compen- 
satory motions  of  eyes  and  head  caused  by  the  orienting 
effect  of  a  moving  retina  image  from  those  caused  by  the 
orienting  effect  of  centrifugal  force  and  this  can  be  done 
easily  in  Phrynosoma. 

When  the  lizard  is  rotated  very  slowly  on  a  turntable 
with  its  eyes  closed,  only  very  slight  compensatory 
motions  of  the  head  and  body  are  observed  during  rota- 
tion, while  very  powerful  compensatory  motions  are  pro- 
duced when  the  motion  of  the  turntable  is  suddenly 
interrupted  after  a  rotation  lasting  about  thirty  seconds. 

When,  however,  the  same  experiment  is  made  with 
the  eyes  of  the  lizard  open  the  reverse  is  observed.  The 
compensatory  motions  of  the  animal  during  rotation  are 
exceedingly  vigorous,  while  the  compensatory  motions  of 
the  animal  after  the  interruption  of  the  rotation  are  slight. 

When  the  eyes  of  the  animal  are  closed  we  are  dealing 
only  with  the  geotropic  effect  of  passive  rotation;  when 
the  eyes  are  open  the  orienting  influence  of  the  moving 
retina  image  is  added  algebraically  to  the  orienting  effect 
of  centrifugal  force  upon  the  ear.  These  two  influences 
act  in  the  same  sense  during  rotation  and  therefore  are 
additive ;  while  after  the  rotation  they  act  in  the  opposite 
sense  to  each  other.  When  we  rotate  the  body  of  an 
animal  passively  to  the  right,  during  rotation  the  objects 
have  an  apparent  motion  to  the  left  and  the  eyes  and  head 
of  the  animal  are  compelled  to  follow  these  moving 
objects,  i.e.,  to  the  left.  The  geotropic  effect  of  passive 
rotation  of  the  animal  to  the  right  also  causes  a  motion 
of  the  eyes  and  head  to  the  left  and  hence  both  effects 
are  additive. 


BHEOTBOPISM  131 

When  a  human  being  has  been  rotated  passively  to  the 
right  for  some  time,  at  the  interruption  of  the  passive 
motion  the  eyes  move  slowly  to  the  right  and  return 
rapidly  to  the  left.  Only  the  slow  motions  give  rise  to 
the  sensation  of  an  apparent  motion  of  the  objects  and 
hence  after  the  sudden  stopping  of  a  passive  rotation 
to  the  right  the  objects  seem  to  such  a  person  to  move 
to  the  left.  The  geotropic  after  effect,  after  passive  rota- 
tion to  the  right,  consists  in  inducing  passive  compensa- 
tory motions  to  the  right,  i.e.,  in  the  opposite  sense  of  the 
orientation  caused  by  the  apparent  motion  of  the  visual 
objects.  Hence  in  the  after  effect  the  orienting  effect 
of  the  retina  image  and  the  centrifugal  effect  weaken 
each  other. 

Lyon321>322>326  has  shown  that  the  phenomena  which 
were  formerly  described  as  rheotropism  in  fish  are  due 
to  the  orienting  effect  of  moving  retina  images.  The 
reader  is  familiar  with  the  fact  that  many  fish  when 
in  a  lively  current  have  a  tendency  to  swim  against  the 
current.  This  phenomenon  was  believed  to  be  due  to 
the  friction  of  the  water.  Lyon  showed  that  fish  orient 
themselves  just  as,  well  when  they  are  put  into  a  closed 
glass  bottle,  which  is  dragged  through  the  water,  although 
in  this  case  they  are  not  under  the  influence  of  any  fric- 
tion from  the  current.  When  the  bottle  is  not  moved 
the  fish  swim  in  any  direction  inside  the  bottle.  It  is 
obviously  the  motion  of  the  retina  images  of  the  objects 
on  the  bank  of  the  brook  which  causes  the  "rheotropic" 
orientation  of  fish.  When  driven  backward  by  the  current 
or  when  dragged  backward  in  a  bottle  through  the  water, 
the  objects  on  the  bank  of  the  river  seem  to  move  in  the 
opposite  direction.  The  animal  being  compelled  to  keep 


132 


TBOPISMS 


the  same  object  fixed,  an  apparent  forward  motion  of  the 
fixed  object  changes  the  muscles  of  the  fins  in  such  a 
sense  as  to  cause  the  animal  to  follow  the  fixed  object 
automatically. 

When  such  rheotropic  fishes  were  kept  in  an  aquarium 
and  a  white  sheet  of  paper  with  black  stripes  was  moved 
constantly  in  front  of  the  aquarium  the  fish  oriented  them- 


1 


fi 


FIG.  39. — Influence  of  motion  of  the  hand  of  an  observer  on  the  direction  of  the  motion 
of  a  swarm  of  sticklebacks  in  an  aquarium.  The  arrows  indicate  the  direction  in  which  the 
hand  was  moved.  The  swarm  of  fish  moves  always  in  the  opposite  direction  in  which  the 
hand  is  moved.  (After  Garrey.) 

selves  against  the  direction  in  which  the  paper  and  its 
stripes  moved.  The  phenomenon  was  more  marked  in 
young  than  in  older  specimens. 

All  the  phenomena  of  rheotropism  ceased  in  the  dark 
or  when  the  fish  were  blind. 

Wheeler  5Q8  has  observed  a  phenomenon  of  anemotrop- 
ism,  namely  that  certain  insects  have  a  tendency  to  put 
the  axis  of  their  body  in  the  direction  of  and  against  the 
wind.  He  considers  this  analogous  to  the  phenomenon  of 
rheotropism  in  fishes.  The  cause  is  also  in  all  probability 
the  tendency  toward  fixation  of  the  moving  retina  image. 

A  very  pretty  demonstration  of  the  orienting  effect 
of  moving  retina  images  was  discovered  by  Garrey  in 


BHEOTKOPISM  133 

sticklebacks.176  When  a  swarm  of  such  fish  was  kept  in 
an  aquarium  it  was  noticed  that  all  the  fish  were  oriented 
with  the  long  axes  parallel  and  that  the  whole  school  swam 
in  a  course  parallel,  but  in  a  direction  opposite,  to  that 
of  the  moving  observer.  If  the  observer  remains  station- 
ary opposite  the  aquarium  and  moves  an  object,  prefer- 
ably white,  which  is  held  in  the  hand,  the  little  fish  at  once 
respond  by  moving  slowly  and  oppositely  to  that  of  the 
moving  object.  They  can  be  thus  made  to  move  up  or 
down  or  to  the  right  or  left  (Fig.  39). 

By  experiments  which  space  forbids  us  to  report  in 
detail  Garrey  has  reached  the  conclusion  that  the  motion 
of  a  near  object  causes  an  apparent  motion  of  the  whole 
horizon  in  the  opposite  direction  and  this  apparent  motion 
the  fish  tries  to  compensate  by  the  motions  of  its  body. 
This  brings  the  observations  on  the  stickleback  into  har- 
mony with  the  general  influence  of  moving  retina  images, 
consisting  in  a  compensatory  motion  of  the  fish. 

We  have  already  referred  to  the  fact  that  the  influence 
of  a  moving  retina  image  is  capable  of  compensating  the 
forced  movement  of  a  dog  after  a  one-sided  lesion  of 
the  cerebral  hemispheres. 


CHAPTER  XV 

STEBEOTROPISM 

OUK  orientation  in  space  is  determined  by  three  groups 
of  tropistic  influences,  two  of  which  we  have  already  dis- 
cussed, light  and  gravitation.  The  third  one  is  pressure 
on  certain  nerve  endings  of  the  skin.  When  the  tactile 
influences  on  the  skin  of  the  soles  of  the  feet  are  weakened 
(as  is  the  case  in  locomotor  ataxia),  the  patient  finds  it 
difficult  to  stand  and  walk  in  the  dark.  When  he  can  use 
his  eyes  the  difficulty  is  diminished,  since  the  orienting 
effect  of  the  retina  image  can  compensate  the  tactile  de- 
ficiency; just  as  we  have  seen  that  the  effect  of  the  loss 
of  the  ears  in  crustaceans  can  be  compensated  by  the 
orienting  influence  of  the  eyes. 

The  role  of  tactile  influences  on  the  orientation  of  ani- 
mals is  most  clearly  demonstrable  in  starfish,  flatworms, 
and  many  other  animals,  when  put  on  their  backs.  The 
animals  "right"  themselves,  i.e.,  they  turn  around  until 
the  ventral  surfaces  or  their  feet  are  pressed  against 
solid  objects  again.  As  the  writer  pointed  out  long  ago,293 
gravitation  has  nothing  to  do  with  the  phenomenon,  since 
starfish  will  stick  to  solid  surfaces  with  their  tube  feet 
even  if  by  so  doing  their  backs  are  permanently  turned 
to  the  center  of  the  earth.  Unless  the  nerve  endings  on  the 
sole  of  their  tube  feet  are  pressed  against  a  solid  surface 
the  animals  are  restless  and  the  arms  move  about  until 
the  feet  are  again  in  contact  with  solid  bodies.  This  phe- 
nomenon of  orientation  the  writer  called  stereotropism. 

Quantitative  investigations  of  this  form  of  tropism  are 

134 


STEREOTROPISM  135 

still  lacking  and  we  must  be  satisfied  with  a  few  descriptive 
remarks. 

Certain  animals  show  a  tendency  to  bring  their  body 
completely  into  contact  with  solid  bodies,  e.g.,  by  creeping 
into  crevices.  Without  further  experimental  test  this 
might  appear  as  an  expression  of  negative  heliotropism, 
but  it  can  be  shown  that  this  assumption  would  be  wrong. 
Amphipyra  is  a  positively  heliotropic  butterfly  which,  in 
spite  of  its  positive  heliotropism,  shows  the  peculiarity 
that  it  creeps  into  crevices  when  given  an  opportunity. 
Such  animals  were  kept  in  a  box  at  the  bottom  of  which 
was  a  square  glass  plate  resting  with  its  four  corners 
on  supports  just  high  enough  to  allow  the  animals  to  creep 
under  the  glass  plate.  After  some  time  every  Amphipyra 
was  found  under  the  glass  plate.  This  happened  also 
when  the  glass  plate  was  exposed  to  full  sunshine,  while 
the  rest  of  the  box  was  in  the  shade.287 

The  same  stereotropism  is  found  in  female  ants  at 
the  time  of  sexual  maturity.  When  such  animals  are  put 
into  a  box  containing  folded  pieces  of  paper  or  of  cloth, 
after  some  time  every  individual  is  found  inside  the  folds. 
This  happens  also  when  the  boxes  are  kept  in  the  dark.287 

The  same  form  of  stereotropism  is  found  in  many 
species  of  worms.  When  earthworms  are  kept  in  jars  with 
vertical  walls  they  are  found  creeping  in  the  corners 
where  their  body  is  as  much  as  possible  in  contact  with 
solid  bodies.  It  is  this  tropism  which  compels  the  animals 
to  burrow  into  the  ground. 

Maxwell349  kept  Nereis,  a  form  of  marine  worms, 
which  burrows  in  sand,  in  a  porcelain  dish  free  from  sand. 
Into  the  dish  glass  tubes  were  put,  whose  diameter  was 
of  the  order  of  that  of  the  worms.  After  24  hours  every 
tube  was  inhabited  by  a  worm  who  made  it  its  permanent 


136  TEOPISMS 

abode.  They  even  remained  in  the  tube  when  exposed 
to  sunlight  which  rapidly  killed  them. 

We  find  the  opposite,  negative  stereotropism,  in  many 
pelagic  animals,  e.g.,  larvae  of  the  barnacle  or  of  other 
crustaceans,  which  avoid  contact  with  solids.  The 
phenomenon  is  liable  to  interfere  with  heliotropic 
experiments. 

The  importance  of  stereotropism  in  animals  was  first 
pointed  out  by  the  experiments  of  Dewitz  on  the  sperma- 
tozoa of  the  cockroach.120, 121  He  noticed  that  when  a 
drop  of  salt  solution  containing  the  spermatozoa  was 
put  under  a  cover  glass  resting  on  low  supports  on  a  slide, 
the  spermatozoa  collect  at  the  solid  surfaces  of  the  slide 
arid  cover  glass,  while  the  liquid  between  remains  free 
from  spermatozoa.  When  a  small  glass  bead  is  put  into 
the  liquid  the  spermatozoa  will  also  swim  on  the  surface 
of  the  bead,  never  leaving  it  again.  Dewitz  is  of  the  opin- 
ion that  this  stereotropism  is  of  assistance  in  securing 
the  entrance  of  a  spermatozoon  into  the  egg.  The  egg  of 
the  cockroach  is  rather  large  and  the  spermatozoon  can 
enter  it  only  through  a  micropyle.  When  the  egg  is  laid 
it  passes  by  the  duct  of  the  seminal  pouch  in  which  the 
female  keeps  the  sperm  after  copulation.  On  passing  the 
duct  some  spermatozoa  reach  the  egg.  Dewitz  points  out 
that  these  cannot  leave  the  surface  of  the  egg  any  more 
but  are  compelled  to  move  incessantly  on  the  surface  of 
the  egg  until  one  of  the  spermatozoa  by  chance  gets  into 
the  micropyle. 

It  is  an  important  fact  that  different  organs  of  the 
same  organism  react  differently.  We  have  already  men- 
tioned the  tendency  of  starfish  or  flatworms  to  right  them- 
selves, i.e.,  their  ventral  surface  is  positively  their  dorsal 
negatively  stereotropic.  The  stolons  of  hydroids  stick 


STEEEOTEOPISM 


137 


to  solid  bodies,  while  the  polyps  bend  and  continue  to  grow 
away  at  right  angles  from  solid  bodies  with  which  they 
come  in  contact.  Thus  the  stem  of  Tubularia  mesem- 
bryanthemum,  a  marine  hydroid,  grows  in  a  straight  line. 
When  such  stems,  after  their  polyp  is  cut  off,  are  put 
with  one  end  in  sand,  the  free  end  forms  a  new  polyp 
and  the  stem  continues  to  grow  in  a  vertical  direction 
upward.  When,  however,  the  stem  is  put  near  the  glass 
wall  as  soon  as  the  polyp  grows  out  it  bends  away  from 


FIG.  40. — The  regenerating  polyp  of  Tubularia  when  in  contact  with  the  glass  wall  of  an 
aquarium  bends  at  right  angles  to  the  glass  wall. 

the  solid  wall,  and  the  stem  will  now  continue  to  grow 
at  right  angles  to  the  vertical  wall  (Fig.  40). 

This  phenomenon  raises  the  question  whether  or  not 
the  law  of  chemical  mass  action  underlies  phenomena 
of  stereotropism.  We  have  seen  that  this  law  dominates 
the  phenomena  of  heliotropism,  inasmuch  as  the  Bunsen- 
Eoscoe  law  is  the  expression  of  the  influence  of  light  on 
the  mass  of  the  photochemical  reaction  product.  We 
have  also  been  able  to  show  that  in  the  case  of  the  geo- 
tropic  curvature  of  Bryopliyllum  the  mass  of  the  apical 


138  TEOPISMS 

leaf  determines  the  rate  of  geotropical  curvature  of  a 
horizontally  placed  stem.  The  only  way  in  which  the 
mass  of  the  leaf  could  have  such  an  influence  is  through 
the  mass  of  substances  it  sends  into  the  stem,  so  that  this 
case  of  geotropism  is  a  function  of  mass  action.  Tnere 
are  indications  that  the  way  contact  with  a  solid  in- 
fluences the  behavior  of  living  matter  is  also  through  the 
influence  on  the  rate  of  certain  chemical  reactions.  The 
writer  observed  that  the  stolons  of  a  hydroid,  Aglao- 
phenia,  have  a  tendency  to  adhere  to  solid  surfaces  and 
not  to  leave  them  any  more  if  they  once  reach  them,  and 
that  as  soon  as  such  a  stolon  reaches  a  solid  surface, 
e.g.,  a  piece  of  a  glass  slide,  its  growth  is  accelerated  con- 
siderably. It  was  very  astonishing  to  notice  how  much 
more  rapid  the  growth  of  roots  of  Aglaophenia  was  when 
they  were  in  contact  with  a  solid  body  than  when  they 
grew  in  sea  water.  The  rate  of  growth  is  the  function  of 
a  chemical  mass  action  (Loeb 543). 


CHAPTER  XVI 

CHEMOTEOPISM 

1.  When  we  create  a  center  of  diffusion  in  water  or  in 
air  we  may  theoretically  expect  orienting  effects.  Thus 
when  a  fine  capillary  tube  containing  a  solution  of  a  salt, 
e.g.,  sodium  malate,  is  put  into  a  drop  of  water  containing 
motile  organisms,  and  the  right  side  of  an  organism  is 
turned  to  the  source  of  diffusion,  the  diffusing  molecules 
will  collect  in  increasing  concentration  on  that  side.  On 
the  left  side  of  the  organism,  no  such  increase  in  the  con- 
centration of  molecules  will  occur.  If  now  the  molecules 
collecting  on  the  right  of  the  organism  in  increasing  den- 
sity are  able  to  produce  some  chemical  or  some  concen- 
tration chain  effect,  the  two  sides  of  the  organism  will  be 
acted  upon  unequally  and  the  tension  of  the  symmetrical 
motile  organs  will  no  longer  be  the  same.  As  a  conse- 
quence the  organism  will  turn  until  the  mass  of  molecules 
or  ions  striking  the  organism  in  the  unit  of  time  will  be 
the  same  for  both  sides.  These  effects  only  take  place 
when  the  organism  is  close  to  the  opening  of  the  capillary 
tube,  since  the  diffusion  from  the  tube  is  slow. 

It  is  obvious,  however,  that  it  is  difficult  to  provide 
experimental  conditions  which  give  exact  chemotropic 
reactions.  First  of  all,  if  the  diffusion  is  rapid  the  differ- 
ences in  concentration  of  the  effective  chemotropic  sub- 
stance on  two  sides  of  an  organism  are  too  slight  to  result 
in  a  turning  movement.  A  second  condition  which  is  liable 
to  vitiate  the  result  are  the  unavoidable  convection  cur- 
rents due  to  changes  or  differences  of  temperature.  In 

139 


140  TBOPISMS 

order  to  get  clear  results  a  method  must  be  used  which 
prevents  a  rapid  diffusion  of  the  substance;  and,  more- 
over, the  current  of  diffusion  must  be  confined  to  an 
almost  straight  line.  It  is  possible  that  Pfeffer's  method 
satisfies  this  condition.424.425  He  introduced  the  sub- 
stance to  be  tested  for  its  chemotropic  effect  into  a  capil- 
lary tube,  the  end  of  which  was  then  sealed.  The  other 
end  was  pushed  into  a  drop  of  water  containing  the  sus- 
pension of  the  organisms  whose  chemotropism  was  under 
investigation.  From  this  capillary  the  diffusion  was  ex- 
tremely slow.  Moreover,  the  current  of  diffusion  was 
approximately  linear  at  the  orifice.  Hence  the  test  for 
the  existence  of  positive  chemotropism  was  perhaps  pos- 
sible. When  an  organism,  struck  sidewise  by  the  line  of 
diffusion  near  the  opening  of  the  capillary  tube,  turns 
toward  the  tube  going  into  it,  some  probability  of  positive 
chemotropism  exists ;  and  when  all  the  organisms  coming 
near  the  orifice  of  the  tube  are  thus  compelled  to  go  into 
it,  the  probability  may  become  certainty,  provided  that 
the  substance  used  does  not  paralyze  the  organism  and 
therefore  act  as  a  trap,  allowing  the  organisms  to  come 
in  but  not  to  go  out.  The  capillary  tubes  used  were  of 
10  to  15  mm.  length  and  of  a  width  of  about  0.1  mm. 
Pfeffer  and  his  pupils  found  that  the  spermatozoa  of 
ferns  go  in  large  numbers  into  a  capillary  tube  containing 
sodium  malate  in  a  concentration  of  0.01  per  cent,  (a  solu- 
tion ten  times  as  diluted  is  still  slightly  active).  This 
effect  of  the  malate  is  specific  in  this  case  and  this  indi- 
cates that  either  a  definite  chemical  action  of  the  malate 
ion  or  a  specific  permeability  of  the  organism  for  it  is  the 
source  of  the  chemotropism.  Such  specific  chemotropic 
effects  are  not  rare,  since  Pfeffer  found  that  Bacterium 
termo  and  Spirillum  undula  are  positively  chemotropic 


CHEMOTEOPISM  141 

to  a  liquid  containing  0.001  per  cent,  of  peptone  or  of 
meat  extract.  It  is  stated  that  cholera  bacilli  are  strongly 
attracted  by  potato  sap.  Pf effer  found  also  that  the  sper- 
matozoa of  certain  mosses  are  positively  chemotropic  to 
cane  sugar  solution  in  dilutions  of  0.1  per  cent. 

Pfeffer's  work  preceded  the  discovery  of  electrolytic 
dissociation,  and  his  pupils  Buller89  and  Shibata465 
made  some  of  the  additions  required  by  the  theory,  namely, 
that  it  is  the  malate  anion  which  acts  in  the  case  of  the 
spermatozoa  of  the  ferns,  and  that  when  the  anion  is 
offered  in  the  form  of  malic  acid  the  H  ion  counteracts 
the  effect  of  the  malate  anion. 

Shibata  made  extensive  experiments  on  the  chemotrop- 
ism  of  the  spermatozoa  of  Isoetes 465  which  he  found  posi- 
tively chemotropic  for  the  malate  anion,  and  also  for  the 
succinate,  tartrate,  and  fumarate  anion,  when  offered  in 
the  form  of  their  neutral  salts.  The  anion  of  the  stereo- 
isomere  of  fumaric  acid,  namely  of  maleic  acid,  was  with- 
out effect.  This  indicates  a  high  degree  of  specificity  of 
these  reactions.  Neutral  sodium  malate  acted  best  in 
dilutions  from  m/100  to  m/1000,  but  some  action  could 
still  be  discovered  in  m/20,000  solutions. 

When  malic  acid  was  used  no  positive  chemotropism 
could  be  discovered  in  solutions  of  m/100  or  above  on 
account  of  the  contrary  effect  of  the  hydrogen  ion,  and 
the  spermatozoa  of  Isoetes  did  not  even  go  into  capillary 
tubes  containing  m/1000  malic  acid.  When  any  acid 
other  than  malic  was  added  to  sodium  malate  the  motion 
of  the  spermatozoa  into  the  tube  was  prevented,  even 
a  m/6000  HC1  solution  still  had  such  an  effect. 

Shibata  studied  especially  the  mode  by  which  the 
spermatozoa  are  oriented  chemotropically  by  malates  and 
found  that  the  reaction  consists  always  in  a  turning  of 


142  TBOPISMS 

the  axis  of  the  body  of  the  spermatozoa  toward  the  capil- 
lary tube  containing  malates  or  succinates,  as  the  tropism 
theory  demands. 

When  the  capillary  tube  and  the  surrounding  medium 
contain  the  same  solute  for  which  the  organisms  are  posi- 
tively chemotropic,  they  will  not  go  into  the  tube  unless 
the  concentration  in  the  tube  is  a  definite  multiple  of  the 
concentration  of  the  outside  solution.  Thus  Pf  effer  found 
that  the  concentration  of  sodium  malate  in  the  capillary 
must  be  at  least  thirty  times  as  great  as  in  the  outside  solu- 
tion to  induce  the  spermatozoa  of  fern  to  move  into  it,  and 
in  the  case  of  Bacterium  termo  the  solution  of  meat  ex- 
tract in  the  tube  had  to  be  at  least  four  times  as  great  as 
the  outside  solution.  In  the  case  of  Isoetes  spermatozoa 
Shibata  found  the  ratio  of  about  400  to  1.  This  constancy 
of  the  ratio  is  known  as  Webpr's  law,  which  therefore 
holds  for  chemotropic  phenomena. 

Lidforss281  found  with  the  aid  of  Pf  effer 's  method 
that  the  spermatozoa  of  Marchantia  are  positively  chemo- 
tropic to  certain  proteins,  especially  egg  albumin,  vitellin 
from  the  egg  yolk,  hemoglobin,  and  mucin  of  the  sub- 
maxillary  gland ;  blood  albumin,  casein,  and  legumin  were 
less  effective.  The  lowest  concentration  for  hemoglobin 
solutions  and  for  egg  albumin  was  0.001  per  cent. ! 

It  may  also  be  stated  that  Lidforss  found  a  chemo- 
tropic effect  of  proteins  upon  the  direction  of  growth  of 
pollen  tubes.280 

Bruchmann81  found  that  the  spermatozoa  of  Ly co- 
podium  were  positively  chemotropic  to  the  watery  extract 
in  which  pieces  of  the  prothallium  had  been  boiled.  Pfef- 
fer's  capillary  method  was  used.  They  showed  also  posi- 
tive chemotropism  to  the  citrate  anion.  Thus,  sodium 
citrate  was  efficient  in  a  0.1  to  0.5  per  cent,  solution.  The 


CHEMOTEOPISM  143 

lower  limit  was  a  little  above  a  0.001  per  cent,  solution. 
The  effect  of  the  free  citric  acid  was  a  mixed  one  since 
the  spermatozoa  were  negative  to  H  ions  and  positive 
to  the  citrate  anion.  Instead  of  being  able  to  use  a  0.1 
per  cent,  solution,  as  in  the  case  of  the  sodium  salt,  a 
0.01  per  cent,  solution  was  the  highest  concentration  to 
which  they  were  positively  chemotropic.  This  means  that 
the  hydrogen  ion  of  citric  acid  solutions  above  m/1000 
repel  the  spermatozoa,  while  when  solutions  of  m/2000 
or  below  are  used  the  hydrogen  ion  effect  no  longer  in- 
hibits the  positive  effect  of  the  citrate  anion.  In  addition 
the  validity  of  Weber 's  law  could  be  demonstrated.  The 
spermatozoa  were  indifferent  to  malates,  oxalates,  and 
many  other  salts,  as  well  as  to  sugar  and  proteins. 

2.  While  all  the  botanical  observers,  from  Buller  on, 
had  found  that  the  hydrogen  ion  has  only  a  preventive 
effect  upon  the  positive  chemotropism  of  lower  organisms, 
Jennings  tried  to  show  that  acids  have  a  positive  effect, 
especially  when  in  low  concentrations.250  But  his  con- 
centrations are  not  quite  as  low  as  he  seems  to  assume, 
since  a  1/50  per  cent,  (m/180)  HC1  solution,  toward  which 
he  believes  to  have  proven  positive  chemotropism  of  Para- 
m&cia,  is  a  deadly  concentrations  Jennings 's  interest  in 
the  problem  was  aroused  by  a  phenomenon  of  aggregation, 
not  infrequently  found  in  the  suspensions  of  infusorians. 

It  is  well  known  that  when  certain  infusoria  are  left  undisturbed 
they  do  not  remain  scattered,  but  gather  in  more  or  less  dense  groups. 
Thus,  if  they  are  mounted  on  a  slide  in  a  thin  layer  of  water,  soon  dense 
aggregations  will  be  formed  in  certain  areas,  while  the  remainder  of  the 

a  The  cells  of  the  stomach  resist  a  much  higher  concentration  of  HC1 
but  this  is  an  exception.  Infusorians,  fish,  and  organisms  in  general  are 
killed  in  a  short  time  in  m/180  HC1  or  in  a  much  lower  concentration  of 
acid.  Thus  Fundulus  does  not  live  more  tham  one  hour  in  m/3000  HC1  or 
HN03.  (Loeb,  J.,  and  Wasteneys,  H.,  Biochem  Z.,  1911,  xxxiii,  489;  1912, 
xxxix,  167.) 


144  TBOPISMS 

slide  will  be  nearly  deserted.  One  of  the  first  investigators  to  describe 
this  phenomenon  was  Pfeffer.  He  observed  its  occurrence  in  Glaucoma 
scintillans,  and  less  markedly  in  Colpidium  colpoda,  Stylonychia  mytilus, 
and  Paramcecium.  Pfeffer  was  inclined  to  believe  that  these  aggregations 
were  due,  partly  at  least,  to  a  contact  stimulus,  resulting  from  a  striking 
of  the  organisms  against  small  solid  bodies,  and  especially  against  each 
other.250 

This  conclusion  of  Pfeffer  may  after  all  be  correct, 
since  it  has  been  shown  that  sea  water  containing  jelly 
from  the  egg  of  a  sea  urchin  causes  spermatozoa  to  stick 
together  for  some  time  when  they  impinge  upon  each 
other.  This  agglutination  no  longer  occurs  when  the 
spermatozoa  are  immobilized.  Jennings  came  to  the  con- 
clusion that  these  aggregations  of  infusorians  are  due  to 
the  fact  that  they  can  go  into  a  weak  concentration  of 
acid,  while  they  cannot  escape  from  such  a  weak  concen- 
tration; and  since  Paramcecia  themselves  produce  C02 
he  assumed  that  the  C02  produced  by  themselves  acts  as 
a  center  of  attraction  for  other  Paramcecia.  In  order  to 
prove  this  he  used  the  following  method : 

The  organisms  were  studied  in  a  thin  layer  of  water,  by  mounting 
them  on  a  slide  covered  with  a  large  cover  glass  supported  near  its 
ends  by  slender  glass  rods.  Their  reactions  were  tested  by  introducing 
with  a  capillary  pipette  a  drop  of  the  substance  in  question  beneath 
the  cover  glass,  or  in  some  cases  by  allowing  it  to  diffuse  inward  from 
the  side  of  the  cover  glass.250 

Thus  Jennings  introduced  a  drop  of  1/50  per  cent. 
(m/180)  HC1  on  a  slide  containing  Chilomonas.  Very 
soon  a  somewhat  denser  ring  of  these  individuals  was 
formed  around  the  drop  (Fig.  41).  A  1/50  per  cent.  HC1 
solution  paralyzes  (and  .soon  kills)  Chilomonas  or  Para- 
mcecia and  hence  the  surface  of  the  drop  must  act  like 
a  trap  into  which  the  organisms  will  steadily  swim,  with- 
out being  able  to  swim  back.  This  will  naturally  increase 


CHEMOTKOPISM  145 

the  density  of  organisms  around  the  drop  and  may  give 
rise  to  a  ring  formation  around  a  high  concentration  of 
HC1  although  the  organisms  are  not  positive  to  the  acid. 
Jennings  found,  however,  that  when  such  organisms  are 
in  a  drop  of  weak  acids  which  do  not  paralyze  the  organ- 
isms quickly,  e.g.,  1/50  per  cent,  acetic  or  in  C02  solutions, 
they  become  negative  to  the  surrounding  neutral  medium 
(H20  or  hay  infusion)  and  stay  in  the  acid.  He,  therefore, 
assumes  that  the  organisms  are  positive  to  weak  acid,  and 


FIG.  41. — Reaction  of  Chilomonas  to  a  drop  of  1/50  per  cent.  HC1.  a,  preparation 
immediately  after  the  introduction  of  the  drop  (no  organisms  either  within  or  gathered 
about  the  drop),  b,  the  same  preparation  a  few  minutes  later.  (After  Jennings.) 

negative  to  strong  acid  as  well  as  to  their  natural  neutral 
or  faintly  alkaline  medium. 

This  negativity  to  their  natural  surroundings  when 
in  weak  acid  as  well  as  to  strong  acid  when  in  weak  acid 
Jennings  does  not  interpret  in  terms  of  the  tropism  theory, 
and  in  this  he  is  probably  correct.  He  interprets  both 
phenomena  as  a  trap  action  due  to  the  asymmetry  of 
certain  infusorians ;  a  sudden  change  in  the  concentration 
of  a  solution  causes  a  reverse  of  the  stroke  of  their  cilia 
by  which  the  organism  is  driven  back.  When  the  old  nor- 
mal stroke  of  the  cilia  is  resumed  the  direction  of  the 
locomotion  is  changed  on  account  of  the  asymmetrical 
arrangement  of  the  cilia.  This  happens  when  the  organ- 
isms go  from  weak  into  strong  acid  or  from  weak  acid  into 
10 


146 


TROPISMS 


a  neutral  medium.  In  this  way  a  collection  of  the  organ- 
isms at  the  surface  of  a  drop  of  acid  may  be  brought 
about.  This  phenomenon  is  not  tropistic  in  the  strict 
sense  of  the  word,  and  as  a  matter  of  fact  Paramcecium  is 
not  positively  chemotropic  to  acid  of  any  strength. 

Barratt24   investigated  the  chemotropism  of  Para- 
mcecia  for  varying  concentrations  of  different  acids  with 

Distilled     Water 


HCL 
0,0001n 


NaOH 
0,001n 


Hay  Infusion 

Fia.  42. — Method  of  proving  that  Paramoecia  are  not  positive  to  acids  of  low  concentration. 

(After  Barratt.) 

Pfeffer's  method  of  capillary  tubes,  counting  the  number 
of  individuals  going  into  the  tube  containing  acid  and 
comparing  it  with  the  number  going  simultaneously  into 
a  control  tube  containing  only  distilled  water  free  from 
C02  (Fig.  42). b  The  acids  used  varied  from  0.001  N  to 
0.0001  N.  The  results  were  unequivocal.  Toward  solu- 
tions of  0.001  N  the  Param&cia  are  negative  and  possibly 

t>  In  addition  two  other  controls  accompanied  the  test,  namely,  one  tube 
containing  hay  infusion  (the  natural  medium  of  the  organisms)  and  one 
alkali. 


CHEMOTBOPISM  147 

also  slightly  negative  to  acids  as  weak  as  0.0001  N.  In  no 
case,  not  even  with  the  weakest  acid,  was  it  possible  to 
prove  the  existence  of  positive  chemotropism  for  acid 
(or  base).  The  number  of  Paramcecia  which  went  into  a 
tube  containing,  e.g.,  0.00002  N  acid,  was  on  the  average 
not  greater  than  that  which  went  into  the  control  tubes. 
The  tubes  were  sufficiently  wide  so  that  the  Paramcecia 
could  and  did  move  into  the  tubes.  Barratt,  therefore, 
concludes  that  acids  have  only  a  repelling  action  upon 
Paramcecia  which,  however,  diminishes  or  disappears 
when  the  hydrogen  ion  concentration  approaches  that  of 
distilled  water. 

The  observations  of  Barratt  contradict  the  statement 
that  Paramcecia  are  positive  to  weak  acid.  We  have  seen 
that  when  spermatozoa  or  swarmspores  are  positive  to 
malates  this  can  be  elegantly  shown  by  Barratt 's  method. 
The  same  method  has  shown  that  when  even  a  trace  of  acid 
is  added  to  the  neutral  malates  this  positivity  disappears. 
By  testing  systematically  all  concentrations  of  different 
acids  within  the  range  to  be  considered,  Barratt  found  no 
trace  of  any  positivity  to  or  any  trap  action  by  weak  acid 
for  Paramcecia.  It  may  be  true,  however,  that  when  the 
organisms  are  in  very  dilute  acid  neutral  or  faintly  alka- 
line water  repels  them  in  the  way  described  by  Jennings. 

Barratt  states  also  that  there  is  nothing  to  support  Jen- 
nings 's  assertion  that  the  C02  given  off  by  the  Paramcecia 
causes  the  aggregation  in  their  natural  medium,  since 
they  are  not  positive  to  low  concentrations  of  hydrogen 
ions.  The  natural  aggregations  of  infusorians  may  be 
due,  as  Pfeffer  suggested,  to  transitory  agglutinations 
when  Paramcecia  impinge  upon  each  other,  and  the  sticki- 
ness or  tendency  to  agglutinate  may  possibly  be  increased 


148  TBOPISMS 

by  certain  substances  produced  and  excreted  by  the  organ- 
isms themselves,  e.g.,  C02. 

3.  The  results  obtained  with  the  spermatozoa  of  ferns 
and  mosses  by  Pfeffer  and  other  botanists  led  some 
authors  to  the  tacit  assumption  that  the  spermatozoa  of 
animals  were  positively  chemotropic  toward  substances 
contained  in  or  secreted  by  the  eggs  of  the  same  species. 
Some  accepted  this  assumption  without  test,  others  made 
tests  which  they  considered  adequate  but  which  seem 
doubtful,  and  it  may  be  of  some  interest  to  discuss  the 
subject,  since  far-reaching  conclusions  might  be  based 
on  these  experiments.  Pfeffer's  method  of  testing  for 
chemotropism  with  the  aid  of  the  capillary  tube  has  proved 
satisfactory  and  the  application  of  this  method  has  shown 
that  the  spermatozoa  of  certain  animals,  e.g.,  of  sea 
urchins,  are  not  chemotropic  toward  substances  contained 
in  or  given  off  by  the  egg.  Thus  Buller,  who  had  worked 
in  Pfeffer's  laboratory  on  the  chemotropism  of  the  sper- 
matozoa of  ferns,  investigated  carefully  and  extensively 
the  question  whether  or  not  the  spermatozoa  of  echino- 
derms  are  positively  chemotropic  for  egg  substances.90 
His  results  were  entirely  negative.  Thoroughly  washed, 
ripe  unfertilized  eggs  of  Arbacia  (Naples)  were  put  into  a 
small  volume  of  sea  water  for  from  2  to  12  hours. 

Capillary  glass  tubes,  about  12  mm.  long  and  0.1  to  0.3  mm.  internal 
diameter,  and  closed  at  one  end,  were  then  half  filled  with  the  (super- 
natant) sea  water  (which  had  contained  the  eggs)  by  means  of  an  air 
pump.  The  tubes  were  then  introduced  into  a  large  open  drop  of  sea 
water,  in  which  fresh,  highly  motile  spermatozoa  were  swimming.  If 
the  eggs  excrete  an  attracting  substance  it  was  argued  that  it  should 
be  present  in  the  tubes,  and  the  spermatozoa  should  collect  there.  .  .  . 
No  attraction  into  the  tube  could  be  observed.  Except  for  a  surface- 
contact  phenomenon  to  be  further  discussed,  they  went  in  and  out  with 
indifference.  Apparently,  therefore,  the  water  which  had  contained  the 
eggs  exercised  no  directive  stimulus  on  the  spermatozoa  whatever. 


CHEMOTROPISM  149 

I  then  attempted  to  find  some  substance  which  could  give  a  ehemo- 
tactic  stimulus  to  spermatozoa.  The  substances  tested  were  such  as  are 
known  to  give  a  directive  chemical  stimulus  to  many  protozoa,  the  sper- 
matozoa of  ferns,  pollen-tubes,  etc.  The  following  solutions  were  tried 
by  the  capillary  tube  method:  distilled  water;  meat  extract  1  per  cent.; 
KNOs  10  per  cent.,  2  per  cent. ;  NaCl  5.8,  2.9,  0.58  per  cent. ;  K*  malate 
1,  0.1  per  cent. ;  asparagin  1  per  cent. ;  glycerine  5  per  cent. ;  grape  sugar 
18,  9,  4.5,  2.25  per  cent. ;  peptone  1  per  cent. ;  alcohol  50,  25, 10  per  cent. ; 
diastase  1  per  cent.;  oxalic  acid  0.9,  0.09,  0.009  per  cent.;  nitric  acid 
1,  0.1,  0.01  per  cent. 

No  definite  chemotactic  reaction — neither  attraction  nor  repulsion — 
was  observed  in  any  case.  Into  tubes  containing  the  weaker  solutions 
the  spermatozoa  went  in  and  out  with  apparent  indifference.  »  .  . 
On  coming  into  contact  with  strong  acid  solutions  (oxalic  acid  0.9,  0.09 
per  cent.;  nitric  acid  1,  0.1  per  cent.)  the  spermatozoa  were  killed,  and 
thus  formed  slight  collections.  They  were  thus  not  able  to  avoid  acids  by 
means  of  a  negative  chemotactic  reaction.90 

Other  authors,  e.g.,  Dewitz  and  the  writer,  have  also 
reached  the  conclusion  that  the  egg  of  the  sea  urchin  con- 
tains no  substance  for  which  the  spermatozoon  of  the 
same  species  is  positively  chemotropic,  and  that  Buller's 
conclusions  that  positive  chemotropism  plays  no  role  in 
the  entrance  of  the  spermatozoon  of  sea  urchins  into  the 
egg  is  correct. 

F.  Lillie  has  recently  expressed  the  opposite  view, 
namely  that  the  egg  of  the  sea  urchin  contains  a  substance 
to  which  the  spermatozoa  are  positively  chemotropic  and 
to  which  he  gave  the  name  ' l  f  ertilizin. ' ' 283  He  first 
tried  Pfeffer's  correct  method  with  capillary  tubes  with 
negative  result,  just  as  Buller  and  the  rest  of  the  obser- 
vers. Instead  of  concluding  that  the  spermatozoa  are 
not  chemotropic  he  discarded  the  method  and  used  Jen- 
nings 's  method,  stating  that  it  gives  "incomparably  more 
delicate  results  than  Pfeffer's  method  of  using  capillary 
tubes "  (p.  533).  Lillie  found  with  this  method  that  the 
spermatozoa  of  Arbacia  are  positively  chemotropic  to 


150  TEOPISMS 

H2S04  of  a  concentration  as  high  as  N/10  and  that  they 
are  never  negatively  chemotropic,  not  even  to  the  highest 
concentrations  of  the  strongest  acid.  It  seems  to  the 
writer  that  Lillie 's  observations  are  more  naturally  ex- 
plained on  the  assumption  that  when  an  acid  is  sufficiently 
strong  and  concentrated,  e.g.,  N/10  HN03  or  H2S04,  it 
will  paralyze  and  kill  the  spermatozoa,  and  that  when 
a  drop  of  such  acid  is  introduced  in  sea  water  containing 
spermatozoa,  a  somewhat  denser  ring  of  the  organisms 
will  be  formed  around  the  surface  of  the  drop  on  account 
of  this  action  of  the  acid. 

With  the  same  method  Lillie  tried  to  prove  that  the 
spermatozoa  of  Nereis  and  Arbacia  are  positively  chemo- 
tropic to  extracts  of  their  own  eggs.283  He  proceeded  as 
follows:  A  suspension  of  Arbacia  sperm,  freshly  made, 
was  put  under  a  raised  cover  slip  and  a  drop  of  the  super- 
natant sea  water  which  had  been  standing  over  eggs  (as 
in  Buller's  experiments)  was  introduced  under  the  cover 
slip.  Observation  with  the  naked  eye  showed  that  around 
this  drop  of  egg-sea  water  immediately  a  dense  ring  of 
spermatozoa  formed  and  behind  this  a  clear  external  zone 
was  formed  about  1.2  to  2  mm.  wide.  The  dense  ring  then 
broke  up  into  small  agglutinated  masses.  In  Lillie 's 
opinion  the  formation  of  this  dense  ring  of  spermatozoa 
at  the  periphery  of  the  egg-sea  water  is  the  expression  of 
a  positive  chemotropism  of  the  spermatozoa  for  a  sub- 
stance contained  in  the  egg-sea  water,  the  ' l  f ertilizin. ' ' 
He  assumes  that  the  spermatozoa  near  the  drop  of  egg- 
sea  water  all  swim  to  the  egg-sea  water,  leaving  a  clear 
space  behind  them.  While  this  explanation  of  the  ring 
formation  might  be  true — if  supported  by  a  direct  chemo- 
tropic method  like  Pf  effer 's — it  can  be  shown  that  the  ring 
formation  is  in  all  probability  due  to  an  entirely  different 


CHEMOTROPISM  151 

phenomenon  which  has  no  relation  to  chemotropism  or 
any  other  tropism.  , 

Buller  had  already  observed  that  the  supernatant  sea 
water  of  sea  urchins  contains  a  substance  which  causes 
the  agglutination  of  spermatozoa.90 

A  drop  of  sea  water  in  which  eggs  had  been  deposited  was  placed 
upon  a  slide  and  a  drop  containing  spermatozoa  near  it.  On  joining 
the  drops  a  large  number  of  small  balls  were  formed  in  a  very  few 
seconds.  When  very  numerous  spermatozoa  were  present  the  balls 
became  0.1  mm.  in  diameter,  containing  many  thousands  of  spermatozoa 
packed  together  in  a  dense  mass. 

Buller  explains  the  phenomenon  as  being  due  to  small 
bits  of  egg  jelly  floating  in  the  sea  water 

so  small  that  they  will  (like  spermatozoa)  pass  through  ordinary  filter 
paper  and,  so  transparent  that  one  cannot  directly  see  them.  A  few 
spermatozoa  become  attached  to  each  piece  of  jelly,  the  presence  of  which 
may  be  inferred  from  the  manner  in  which  the  small  groups  of  sperma- 
tozoa move  about.  Owing  to  the  length  of  the  spermatozoon,  although  its 
head  may  be  imbedded  in  a  jelly  particle,  the  tail  may  remain  partly  free. 
The  little  collections  of  spermatozoa  thus  move  about  hither  and  thither 
in  no  particular  direction.  When  two  such  groups  come  by  accident 
into  contact  they  fuse.  Certain  of  the  spermatozoa  adhere  to  both  little 
masses  of  jelly  and  lock  them  together.  The  fused  mass  combines  with 
other  simple  and  fused  masses,  and  so  on.c 

The  writer  was  able  to  show  that  when  the  jelly  of  the 
egg  of  Strongylocentrotus  purpuratus  is  dissolved  by  an 
acid  treatment  the  eggs  when  washed  and  transferred 
to  sea  water  no  longer  give  off  agglutinating  substances, 
while  the  acid  sea  water  containing  the  dissolved  jelly, 
when  rendered  neutral  through  the  addition  of  alkali,  will 
cause  the  agglutination  of  sperm.302  While  all  the  jelly 
can  be  washed  off  with  an  acid  treatment  in  the  egg  of 
purpuratus,  the  same  is  not  true  for  the  egg  of  Arbacia 

c  This  explanation  of  the  fusion  of  two  clusters  to  a  larger  one  is  per- 
haps not  correct.  The  writer  is  inclined  to  ascribe  it  to  the  adhesion  or 
agglutination  of  the  spermatozoa  of  two  neighboring  clusters  with  each 
other,  due  to  a  sticky  surface  on  the  sperm  head, 


152  TEOPISMS 

of  Woods  Hole.  Here  the  acid  treatment  does  not  as  a  rule 
dissolve  all  the  jelly,  or  possibly  some  new  jelly  may  be 
given  off  by  the  egg. 

While  Buller  may  be  correct  in  assuming  that  micro- 
scopic pieces  of  the  egg  jelly  form  the  center  of  these 
sperm  clusters,  the  writer  reached  the  conclusion  that 
the  dissolved  mass  of  the  jelly  makes  the  surface  of  the 
spermatozoa  transitorily  sticky,  so  that  if  they  impinge 
against  each  other  they  will  stick  together  for  some  time, 
until  the  sticky  compound  formed  by  the  jelly  on  the  sperm 
head  is  dissolved  by  the  sea  water,  which  occurs  after  a 
short  time. 

This  agglutinating  effect  of  the  egg-sea  water  upon 
the  sperm  of  Arbacia  gives  rise  to  that  ring  formation 
which  Lillie  considers  a  proof  of  positive  chemotropism. 
When  a  drop  of  egg-sea  water  is  put  into  a  sufficiently 
dense  suspension  of  spermatozoa,  the  spermatozoa  at  the 
surface  of  the  drop  will  agglutinate  into  practically  one 
dense  ring  around  it,  and  through  the  diffusion  of  some 
of  the  dissolved  jelly  through  this  ring  numerous  little 
clusters  will  form  at  the  external  periphery  of  the  ring, 
and  these  clusters  will  fuse  with  the  ring.  In  this  way 
the  clear  region  behind  the  ring  originates.  The  process 
of  fusion  continues  inside  the  ring  with  the  result  that 
the  latter  breaks  up  into  numerous  bead-like  spherical 
clusters  as  Lillie  described.  In  a  former  paper  the  writer 
has  pointed  out  the  analogy  between  the  phenomena  of 
transitory  sperm  agglutination  (under  the  influence  of 
egg-sea  water)  and  surface  tension  phenomena,  inasmuch 
as  two  small  clusters  upon  coming  in  contact  fuse  into  one 
larger  one  and  inasmuch  as  elongated  clusters  break  up 
into  two  or  more  spherical  clusters. 

The  ring  formation  described  by  Lillie  has,  therefore, 


CHEMOTKOPISM  153 

in  the  opinion  of  the  writer  no  connection  with  positive 
chemotropism.d 

4.  The  method  of  Pfeffer  cannot  well  be  used  for 
larger  organisms.  Barrows  25  has  devised  an  apparatus 
which  allowed  him  to  test  quantitatively  the  chemotropic 
reactions  of  Drosophila.  The  flies  which  are  positively 
heliotropic  were  allowed  to  go  to  the  light  inside  of  a 
narrow  hollow  groove.  At  a  certain  spot  of  the  groove 
two  glass  bottles  were  inserted  with  their  openings  oppo- 
site each  other,  one  of  which  contained  the  substance  to 
be  tested  for  chemotropic  efficiency,  while  the  other  served 
as  a  control.  The  number  of  flies  which  on  their  path 
were  deviated  by  the  bottle  containing  the  substance  to 
be  tested  were  counted  and  their  number  compared  with 
that  going  into  the  control  bottle.  The  collection  of  odor- 
ous matter  in  the  groove  was  removed  by  suction.  In  this 
way  it  was  possible  to  ascertain  that  the  flies  are  posi- 
tively chemotropic  to  ethyl  and  amyl  alcohol,  acetic  and 
lactic  acid,  and  to  ether.  The  chemotropic  effect  of  alcohol 
was  increased  through  the  admixture  of  traces  of  an  ester, 
e.g.,  methyl  acetate. 

> ;  In  describing  the  manner  of  reaction  of  these  flies, 
Barrows  makes  the  statement  that  when  the  odor  is  weak 
the  fruit  fly  i '  attempts  first  to  find  the  food  by  the  method 
of  trial  and  error,  but  as  the  fly  passes  into  an  area  of 
greater  stimulation,  these  movements  give  way  to  a  direct 
orientation.  This  orientation  is  a  well  defined  tropism 
response."  A  similar  statement  had  been  made  by 

d  Lillie  also  assumes  that  it  is  the  intensity  gradient  which  determines 
the  direction  of  motion  in  tropistic  reactions.  This  is  not  correct,  since  posi- 
tively heliotropic  animals  go  to  the  light  even  if  by  so  doing  they  have  to  go 
from  strong  into  weak  light  ( see  page  50 ) .  The  direction  of  motion  in 
tropistic  reactions  is  determined  by  differences  in  the  mass  of  chemical 
substances  on  both  sides  of  a  symmetrical  animal. 


154  TBOPISMS 

Harper  for  the  heliotropism  of  certain  worms,  namely 
that  in  strong  light  the  animals  move  by  heliotropism, 
in  weak  light  by  " trial  and  error."  These  statements 
are  as  erroneous  as  the  assertion  that  while  a  stone  falls 
under  the  influence  of  gravity  a  feather  finds  its  way  down 
by  the  method  of  ' '  trial  and  error. ' ' 

Barrows  and  Harper  overlook  the  role  of  mass  action 
and  reaction  velocity.  When  an  animal  is  struck  on  one 
side  only  by  light  or  by  a  chemically  active  substance 
emanating  from  a  center  of  diffusion,  the  mass  of  this 
substance  or  of  the  photochemical  reaction  product  in- 
creases on  this  side.  These  substances  react  with  some 
substance  of  the  nerve  endings  and  as  soon  as  the  mass 
of  the  reaction  product  reaches  a  certain  quantity  the 
automatic  turning,  the  tropistic  reaction,  occurs.  When 
the  light  is  strong  or  when  the  animal  is  near  the  center 
of  diffusion,  this  happens  in  a  short  time  and  the  tropistic 
character  of  the  reaction  is  striking,  since  the  animal  is 
quickly  put  back  into  its  proper  orientation  if  it  deviates 
from  it.  When  the  light  is  weak  or  when  the  animal  is  at 
some  distance  from  the  center  of  diffusion  it  will  take  a 
longer  time  before  this  critical  value  of  the  reaction  prod- 
uct is  reached,  and  in  this  case  the  animal  can  deviate 
considerably  out  of  the  correct  orientation  before  it  is 
brought  back  into  the  right  orientation. 


CHAPTER  XVII 

-V 

THEEMOTROPISM 

UNDER  the  name  of  thermotropism  M.  Mendels- 
sohn 352-355  has  described  the  observation  that  Paramcecia 
gather  at  a  definite  end  of  a  trough  when  these  ends  have 
a  different  temperature.  The  organisms  were  put  into  a 
flat  trough  resting  on  tubes  through  which  water  was 
flowing.  When  the  water  in  the  tube  had  a  temperature 
of  38°  at  one  end  of  the  trough,  while  the  tube  at  the 
opposite  end  was  perfused  by  water  of  26°  the  organisms 
all  gathered  at  the  latter  end.  If  then  the  temperature 
of  the  water  in  the  two  tubes  was  reversed  the  organisms 
went  to  the  other  end  of  the  trough.  If  one  end  had  the 
temperature  of  10°  the  other  of  25°,  all  went  to  the  latter 
end.  In  this  case  we  are  in  all  probability  not  dealing  with 
a  tropistic  reaction  but  with  a  collection  of  organisms  due 
to  the  mechanism  of  motion  described  for  Param&cium 
by  Jennings.  When  these  organisms  come  suddenly  from 
a  region  of  a  moderate  temperature  to  one  of  lower  tem- 
perature the  activity  of  their  cilia  is  transitorily  reversed, 
but  owing  to  the  asymmetrical  arrangement  of  their  cilia 
they  do  not  go  back  in  the  old  direction  but  deviate  to  one 
side.  This  can  lead  to  a  collection  of  Paramczcia  such  as 
Mendelssohn  described. 

- 


155 


CHAPTER  XVIII 

INSTINCTS 

THE  teleological  way  of  analyzing  animal  conduct  has 
predominated  to  such  an  extent  that  there  has  been  a 
tendency  to  connect  all  animal  reactions  with  the  preser- 
vation of  the  individual  and  the  species.  Instincts  are 
considered  to  be  such  reactions  of  the  organism  as  a  whole 
which  lead  to  the  nutrition  of  the  individual,  the  mating 
of  the  two  sexes,  and  the  care  of  the  offspring.  "If  the 
tropism  theory  of  animal  conduct  is  justified  it  must  be 
possible  to  show  that  instincts  are  tropistic  reactions. 

We  have  insisted  in  previous  chapters  that  animals 
indifferent  to  light  can  be  made  strongly  positively  or 
negatively  heliotropic  by  certain  chemicals  or  vice  versa 
(e.g.,  the  experiments  on  certain  fresh  water  crustaceans 
with  acids  or  alcohol  and  caffein) .  We  know  that  the  body 
itself  produces  at  various  periods-  of  its  existence  definite 
hormones  and  such  hormones  can  act  similarly  as  the  acids 
or  the  caffein  in  the  experiments  on  crustaceans,  since  it 
makes  no  difference  whether  such  substances  as  acid  are 
introduced  into  the  blood  from  the  outside  or  from  certain 
tissues  of  the  animal's  own  body.  We  know  through 
F.  Lillie  's  observations  that  in  the  blood  of  the  male  cattle 
embryo  substances  circulate  which  inhibit  the  develop- 
ment of  secondary  sexual  characters  of  the  female  embryo, 
and  we  know  through  Steinach's  experiments  that  the  in- 
termediate tissue  from  the  sexual  gland  of  one  sex  when 
introduced  into  the  castrated  organism  of  the  opposite 
sex  may  impart  to  the  latter  the  sexual  instincts  of  the 

156 


INSTINCTS  157 

former.  Hormones  produced  by  definite  tissues,  there- 
fore, influence  the  instincts.  We  want  to  show  that  this 
influence  is  due  to  a  modification  of  tropistic  reactions 
by  the  hormones. 

Mating  in  certain  fish,  like  Fundulus,  consists  in  the 
male  pressing  that  part  of  its  body  which  contains  the 
opening  of  the  sperm  duct  against  the  corresponding  part 
of  the  female  body.  The  latter  responds  by  pressing  back, 
and  the  pressure  of  the  body  is  maintained  by  both  sexes 
through  motions  of  the  tail.  During  this  mutual  pressure 
or  friction  both  sexes  shed  their  sexual  cells,  sperm  and 
eggs,  into  the  water,  and  since  the  openings  of  the  cloaca 
of  the  male  and  female,  through  which  the  sex  cells  are 
shed,  are  brought  almost  in  contact  with  each  other,  sperm 
and  eggs  mix  at  the  moment  they  are  shed.  This  act  of 
mating  is  due  to  a  stereotropism  which  exists  only  during 
the  spawning  season  and  which  is  supposedly  due  to  cer- 
tain hormones  existing  at  this  time  in  the  animal.  The 
existence  of  such  hormones  is  also  indicated  by  certain 
colorations  which  develop  and  exist  in  the  male  during 
this  period.  This  stereotropism  is  to  some  extent  specific 
since  it  is  exhibited  by  the  contact  between  the  two  sexes. 
The  specificity  of  this  stereotropism  is  of  importance 
and  needs  further  experimental  analysis,  but  that  it  is 
in  reality  a  type  of  common  stereotropism  is  evidenced 
by  the  fact  that  if  during  the  spawning  season  we  keep 
females  isolated  from  males  in  an  aquarium  the  females 
will  go  through  the  motions  of  mating  and  shed  the  eggs 
every  time  they  come  in  contact  with  the  glass  walls  of  the 
aquarium.  When  they  are  kept  permanently  isolated  from 
the  male  they  repeat  this  non-specific  purely  stereotropic 
mating  throughout  the  season.  The  eggs  which  they  shed 
they  quite  frequently  devour. 


158  TEOPISMS 

These  manifestations  of  a  highly  developed  stereo- 
tropism  in  the  segments  of  the  reproductive  organs  are 
probably  widespread  in  the  animal  kingdom.  The  late 
Professor  Whitman  told  the  writer  that  male  pigeons 
when  kept  in  isolation  will  try  to  go  through  the  motions 
of  mating  with  any  solid  object  in  their  field  of  vision, 
e.g.,  glass  bottles,  and  even  with  objects  which  give  only 
the  optical  impression  of  a  solid,  namely,  their  own 
shadow  on  the  ground. 

In  ants,  the  winged  males  and  females  become  intensely 
positively  heliotropic  at  the  time  of  mating.  Copulation 
occurs  in  the  air,  in  the  so-called  nuptial  flight.  At  a  cer- 
tain time — in  the  writer's  observation  toward  sunset, 
when  the  sky  is  illuminated  at  the  horizon  only — the  whole 
swarm  of  males  and  females  leave  the  nest  and  fly  in  the 
direction  of  the  glow.  The  wedding  flight  is  a  heliotropic 
phenomenon287  presumably  due  to  substances  produced 
in  the  body  during  this  period.  After  copulation  the 
female  loses  its  wings  and  also  its  positive  heliotropism.a 
It  becomes  now  intensely  stereotropic.  When  kept  in  a 
dark  box  with  pieces  of  cloth  in  folds  the  wingless  female 
will  now  be  found  in  the  folds  where  its  body  is  as  closely 
as  possible  in  contact  with  the  solids.  This  positive 
stereotropism  leads  the  queen  to  begin  a  subterranean 
existence  which  marks  the  founding  of  a  new  nest.  Helio- 
tropism  and  stereotropism  are,  therefore,  the  controlling 
factors  in  mating  and  the  starting  of  a  new  nest  in  these 
ants.287 

V.  L.  Kellogg 265  has  made  observations  which  show 
that  the  nuptial  flight  in  bees  is  also  due  to  an  outburst 
of  positive  heliotropism  as  in  the  ant. 

a  It  has  already  been  mentioned  that  artificial  removal  of  the  wings  of 
the  fruit  fly  will  also  abolish  its  heliotropism. 


INSTINCTS  159 

In  the  course  of  some  experiments  on  the  sense-reactions  of  honey- 
bees, I  liave  kept  a  small  community  of  Italian  bees  in  a  glass-sided, 
narrow,  high  observation  hive,  so  made  that  any  particular  bee,  marked, 
which  it  is  desired  to  observe  constantly,  can  not  escape  this  obser- 
vation. The  hive  contains  but  two  frames,  one  above  the  other,  and 
is  made  wholly  of  glass,  except  for  the  wooden  frame.  It  is  kept  covered, 
except  during  observation  periods,  by  a  black  cloth  jacket.  The  bees  live 
contentedly  and  normally  in  this  small  hive,  needing  only  occasional 
feeding  at  times  when  so  many  cells  are  given  up  for  brood  that  there 
are  not  enough  left  for  sufficient  stored  food  supplies.  Last  spring 
at  the  normal  swarming  time,  while  standing  near  the  jacketed  hive, 
I  heard  the  excited  hum  of  a  beginning  swarm  and  noted  the  first  issuers 
rushing  pellmell  from  the  entrance.  Interested  to  see  the  behavior  of 
the  community  in  the  hive  during  such  an  ecstatic  condition  as  that  of 
swarming,  I  lifted  the  cloth  jacket,  when  the  excited  mass  of  bees  which 
was  pushing  frantically  down  to  the  small  exit  in  the  lower  corner  of 
the  hive  turned  with  one  accord  about  face  and  rushed  directly  upward 
away  from  the  opening  toward  and  to  the  top  of  the  hive.  Here  the 
bees  jammed,  struggling  violently.  I  slipped  the  jacket  partly  on;  the 
ones  covered  turned  down;  the  ones  below  stood  undecided;  I  dropped 
the  jacket  completely;  the  mass  began  issuing  from  the  exit  again; 
I  pulled  off  the  jacket,  and  again  the  whole  community  of  excited  bees 
flowed — that  is  the  word  for  it,  so  perfectly  aligned  and  so  evenly 
moving  were  all  the  individuals  of  the  bee  current — up  to  the  closed  top 
of  the  hive.  Leading  the  jacket  off  permanently,  I  prevented  the  issuing 
of  the  swarm  until  the  ecstasy  was  passed  and  the  usual  quietly  busy 
life  of  the  hive  was  resumed.  About  three  hours  later  there  was  a 
similar  performance  and  failure  to  issue  from  the  quickly  unjacketed 
hive.  On  the  next  day  another  attempt  to  swarm  was  made,  and  after 
nearly  an  hour  of  struggling  and  moving  up  and  down,  depending  on 
my  manipulation  of  the  black  jacket,  most  of  the  bees  got  out  of  the 
hive's  opening  and  the  swarming  came  off  on  a  weed  bunch  near  the 
laboratory.  That  the  issuance  from  the  hive  at  swarming  time  depends 
upon  a  sudden  extra-development  of  positive  heliotropism  seems  obvious. 
The  ecstasy  comes  and  the  bees  crowd  for  the  one  spot  of  light  in  the 
normal  hive,  namely,  the  entrance  opening.  But  when  the  covering 
jacket  is  lifted  and  the  light  comes  strongly  in  from  above — my  hive 
was  under  a  skylight — they  rush  toward  the  top,  that  is,  toward  the  light. 
Jacket  on  and  light  shut  off  from  above,  down  they  rush;  jacket  off 
and  light  stronger  from  above  than  below  and  they  respond  like  iron 
filings  in  front  of  an  electromagnet  which  has  its  current  suddenly 
turned  on. 


160  TBOPISMS 

Finally  there  are  indications  of  the  role  of  chemo- 
tropism  in  mating.  It  has  been  observed  for  a  long  time 
that  if  a  female  butterfly  is  kept  hidden  from  sight  in  a 
not  too  tightly  closed  box,  male  butterflies  of  the  same 
species  will  be  attracted  by  the  box  and  settle  on  it.  The 
female  apparently  gives  off  a  substance  to  which  the  male 
is  positively  chemotropic.  All  these  observations  should 
be  worked  out  more  systematically.  The  data  suffice, 
however,  to  indicate  that  what  the  biologist  and  psycholo- 
gist call  instinct  are  manifestations  of  tropisms. 

The  fact  that  eggs  are  laid  by  many  insects  on  material 
which  serves  as  a  nutritive  medium  for  the  offspring  is  a 
typical  instinct.  An  experimental  analysis  shows  again 
that  the  underlying  mechanism  of  the  instinct  is  a  positive 
chemotropism  of  the  mother  insect  for  the  type  of  sub- 
stance serving  her  as  food ;  and  when  the  intensity  of  these 
volatile  substances  is  very  high,  i.e.,  when  the  insect  is  on 
the  material,  the  egg-laying  mechanism  of  the  fly  is  auto- 
matically set  into  motion.  Thus  the  common  housefly 
will  deposit  its  eggs  on  decaying  meat  but  not  on  fat; 
but  it  will  also  deposit  it  on  objects  smeared  over  with 
asafotida,  on  which  the  larvae  cannot  live.  Aseptic  banana 
flies  will  lay  their  eggs  on  sterile  banana,  although  the 
banana  is  only  an  adequate  food  for  the  larvae  when  yeast 
grows  on  it.  It  seems  that  the  female  insect  lays  her  eggs 
on  material  for  which  she  is  positively j3J3£motropic,  and 
this  is  generally  material  which  she  also  eats.  The  fact 
that  such  material  serves  as  food  for  the  coming  genera- 
tion is  an  accident.  Considered  in  this  way,  the  mystic 
aspect  of  the  instinctive  care  of  insects  for  the  future 
generation  is  replaced  by  the  simple  mechanistic  concep- 
tion of  a  tropistic  reaction.  yln  this  case  natural  selection 
plays  a  role  since  species  whose  females  would  too  fre- 


INSTINCTS  161 

quently  lay  their  eggs  on  material  on  which  the  larvae 
cannot  thrive  would  be  liable  to  die  out.  ^) 

As  an  illustration  of  the  role  of  tropisins  in  the  instinc- 
tive self-preservation  the  writer  wishes  to  apologize  for 
selecting  an  example  which  he  has  used  so  often  in  pre- 
vious discussions,  namely  the  role  of  heliotropism  in  the 
preservation  of  the  life  of  the  caterpillars  of  Porthesia 
chrysorrhcea.287  This  butterfly  lays  its  eggs  upon  a  shrub, 
on  which  the  larvae  hatch  in  the  fall  and  on  which  they 
hibernate,  as  a  rule,  not  far  from  the  ground.  As  soon 
as  the  temperature  reaches  a  certain  height,  they  leave  the 
nest ;  under  natural  conditions  this  happens  in  the  spring 
when  the  first  leaves  have  begun  to  form  on  the  shrub. 
(The  larvae  can,  however,  be  induced  to  leave  the  nest  at 
any  time  in  the  winter,  provided  the  temperature  is  raised 
sufficiently).  After  leaving  the  nest,  they  crawl  directly 
upward  on  the  shrub  where  they  find  the  leaves  on  which 
they  feed.  If  the  caterpillars  should  move  down  the  shrub 
they  would  starve,  but  this  they  never  do,  always  crawl- 
ing upward  to  where  they  find  their  food.  What  gives 
the  caterpillar  this  never-failing  certainty  which  saves 
its  life  and  for  which  the  human  being  might  envy  the 
little  larva!  Is  it  a  dim  recollection  of  experiences  of 
former  generations,  as  Samuel  Butler  would  have  us 
believe !  It  can  be  shown  that  this  instinct  is  merely  posi- 
tive heliotropism  and  that  the  light  reflected  from  the  sky 
guides  the  animals  upward.  The  caterpillars  upon  waking 
from  their  winter  sleep  are  violently  positively  heliotropic, 
and  it  is  this  heliotropism  which  makes  the  animals  move 
upward.  At  the  top  of  the  branch  they  come  in  contact 
with  a  growing  bud  and  chemical  andjactile  influences  set 
the  mandibles  of  the  young  caterpillar  into  activity.  If  we 
put  these  caterpillars  into  closed  test  tubes  which  lie 
11 


162  TBOPISMS 

with  their  longitudinal  axes  at  right  angles  to  the  window 
they  will  all  migrate  to  the  window  end  where  they  will 
stay  and  starve,  even  if  we  put  their  favorite  leaves  into 
the  test  tube  close  behind  them.  These  larvae  are  in  this 
condition  slaves  of  the  light. 

The  few  young  leaves  on  top  of  a  twig  are  quickly  eaten 
by  the  caterpillar.  The  light  which  saved  its  life  by  mak- 
ing it  creep  upward  where  it  finds  its  food  would  cause 
it  to  starve  could  the  animal  not  free  itself  from  the 
bondage  of  positive  heliotropism.  After  having  eaten  it 
is  no  longer  a  slave  of  light  but  can  and  does  creep  down- 
ward. It  can  be  shown  that  a  caterpillar  after  having 
been  fed  loses  its  positive  heliotropism  almost  completely 
and  permanently.  If  we  submit  unfed  and  fed  caterpillars 
of  the  same  nest  to  the  same  artificial  or  natural  source  of 
light  in  two  different  test  tubes  the  unfed  will  creep  to 
the  light  and  stay  there  until  they  die,  while  those  that 
have  eaten  will  pay  little  or  no  attention  to  the  light. 
Their  positive  heliotropism  has  disappeared  and  the  ani- 
mal after  having  eaten  can  creep  in  any  direction.  The 
restlessness  which  accompanies  the  condition  of  starva- 
tion makes  the  animal  leave  the  top  of  the  branches  and 
creep  downward — which  is  the  only  direction  open  to  it— 
where  it  finds  new  young  leaves  on  which  it  can  feed.  The 
wonderful  hereditary  instinct  upon  which  the  life  of  the 
/  animal  depends  is  its  positive  heliotropism  in  the  unfed 
condition  and  the  loss  of  this  heliotropism  after  having 
eaten.  The  chemical  changes  following  the  taking  up 
of  the  food  abolish  the  heliotropism  just  as  C02  arouses 
positive  heliotropism  in  certain  Daphnia. 

Mayer  and  Soule  have  shown  that  negative  geotropism 
and  positive  heliotropism  keep  the  caterpillars  of  Danais 
plexippus  on  its  plant  (the  milk-weed).  The  chemical 


INSTINCTS  163 

nature  of  the  leaf  starts  the  eating  reactions,  but  "once 
the  eating  reaction  be  set  into  play,  it  tends  to  continue, 
so  that  the  larva  may  then  be  induced  to  eat  substances 
which  it  would  never  have  commenced  to  eat  in  the  first 
instance.  "3fil 

These  few  examples  may  suffice  to  show  that  the  theory 
of  tropisms  is  at  the  same  time  the  theory  of  instincts  if 
due  consideration  is  given  to  the  role  of  hormones  in 
producing  certain  tropisms  and  suppressing  others.  A 
systematic  analysis  of  instinctive  reactions  from  the  view- 
point of  the  theory  of  tropisms  and  hormones  will  prob- 
ably yield  rich  returns.  As  an  example  we  may  quote  the 
fact  that  diurnal  depth  migrations  of  aquatic  animals, 
consisting  in  an  upward  motion  during  the  night  and  a 
downward  motion  during  the  day,  are  in  all  probability 
determined  by  a  periodic  change  in  the  sense  of 
heliotropism.183'  30° 


CHAPTER  XIX 

MEMOKY  IMAGES  AND  TROPISMS 

WHEN  a  muscle  is  stimulated  several  times  in  succes- 
sion, the  effect  of  the  second  or  third  or  later  stimulation 
may  be  greater  than  that  of  the  first.  A  consistently 
anthropomorphic  author  should  draw  the  inference  that 
the  muscle  is  gradually  learning  to  react  properly.  What 
seems  to  happen  is  that  the  hydrogen  ion  concentration  is 
raised  by  the  first  stimulations  to  a  point  where  the  effect 
of  the  stimulation  becomes  greater.  When  the  stimula- 
tions continue  and  the  hydrogen  ion  concentration  be- 
comes still  greater,  the  response  of  the  muscle  declines  and 
finally  becomes  zero ;  the  hydrogen  ion  concentration  has 
now  become  too  high.  The  writer  observed  that  when 
winged  plant  lice  of  a  Cineraria  were  taken  directly  from 
the  plant,  they  did  not  react  as  promptly  as  after  they 
had  gone  through  several  heliotropic  experiments.  There 
is  nothing  to  indicate  that  this  is  a  case  of  "  learning, " 
since  it  may  also  be  the  result  of  a  change  in  the  hydrogen 
ion  concentration  or  of  some  other  reaction  product.  It 
may  also  be  the  result  of  some  purely  mechanical  obstacle 
to  rapid  locomotion  being  removed. 

We  can  speak  of  learning  only  in  such  organisms  in 
which  the  existence  of  associative  memory  can  be  proved. 
By  associative  memory  we  mean  that  mechanism,  by 
which  a  stimulus  produces  not  only  the  direct  effects 
determined  by  its  nature,  but  also  the  effects  of  entirely 
different  stimuli  which  at  some  former  period  by  chance 
attacked  the  organism  at  the  same  time  with  the  given 

164 


MEMORY  IMAGES  165 

stimulus.  Thus  the  image  or  the  odor  of  a  rose  may  call 
up  the  memory  of  persons  or  surroundings  which  were 
present  on  a  former  occasion  when  the  image  or  odor  of 
the  flower  impressed  us.  Brain  physiology  shows  that 
this  type  of  associative  memory  is  the  specific  function 
of  definite  parts  of  the  brain,  e.g.,  the  cerebral  hemispheres 
which  exist  only  in  definite  types  of  animals.  We  see 
also  that  certain  species  among  vertebrates,  insects,  crus- 
tacea,  and  cephalopods  possess  associative  memory,  while 
to  the  knowledge  of  the  writer  no  adequate  proof  for  its 
existence  has  ever  been  given  for  worms,  starfish,  sea 
urchins,  actinians,  medusae,  hydroids,  or  infusorians.293 
Claims  for  the  existence  of  such  memory  in  these  latter 
groups  of  animals  have  frequently  been  made,  but  such 
claims  are  either  plain  romance  or  due  to  a  confusion  of 
reversible  physiological  processes  with  the  irreversible 
phenomena  of  associative  memory.  The  less  a  scientist 
is  accustomed  to  rigid  quantitative  experiments,  the  more 
ready  he  is  to  confound  the  reversible  after  effects  of  a 
stimulus — e.g.,  the  after  effects  due  to  an  increase  in 
hydrogen  ion  concentration — with  indications  of  associa- 
tive memory.  Learning  is  only  possible  where  there  exists  v 
a  specific  organ  of  associative  memory,  the  physical 
mechanism  of  which  is  still  unknown. 

The  manifestations  of  associative  memory  are  gener-  \/ 
ally  discussed  by  the  introspective  psychologists,  who  as 
a  rule  are  not  familiar  with  or  do  not  appreciate  the 
methods  of  the  physicist.  There  have  been  made  repeated 
attempts  to  develop  methods  for  the  analysis  of  associa- 
tive memory,  among  which  thus  far  only  one  satisfies  the 
demands  of  quantitative  science,  namely  Pawlow's 
method.  As  is  well  known  even  to  the  layman,  eating 
causes  a  flow  of  saliva.  The  quantity  of  saliva  excreted 


166  TROPISMS 

by  the  parotid  (one  of  the  salivary  glands)  in  the  dog 
can  be  collected  and  measured.  The  earlier  physiological 
workers  had  observed  that  in  a  dog  which  had  often  been 
used  for  the  study  of  the  influence  of  eating  upon  the  flow 
of  saliva,  the  saliva  began  to  flow  whenever  the  prepara- 
tions for  feeding  were  made  before  the  eyes  of  the  dog, 
even  when  no  food  was  given.  Pawlow  made  use  of  this 
fact  to  study  quantitatively  the  "strength"  of  such  asso- 
ciative phenomena,  which  he  terms  "conditioned  re- 
flexes" (to  escape  the  terminology  and  interpretations 
of  the  introspective  psychologist).537  A  fistula  a  of  the 
duct  of  the  par  otic  gland  allows  the  saliva  to  flow  outside 
the  cavity  of  the  mouth.  This  fistula  is  connected  with  a 
long  manometer  which  by  a  special  air  chamber  arrange- 
ment gives  a  considerable  change  in  the  height  of  the 
meniscus  for  the  secretion  of  as  little  as  one  drop  of 
saliva,  The  variations  of  the  height  of  the  column  of 
liquid  in  the  manometer  are  observed  outside  of  the  room 
where  the  dog  is.  For  each  dog  which  is  to  serve  for 
such  experiments  the  meal  is  preceded  by  a  certain  signal, 
the  sounds  of  a  metronome  of  definite  rhythm,  or  a  definite 
musical  sound,  or  a  definite  optical  signal,  and  so  forth, 
which  is  to  form  the  special  conditioned  reflex  for  this  dog. 
After  a  certain  number  of  repetitions  the  association  is 
established  and  from  now  on  the  flow  of  saliva  commences 
from  the  dog's  parotid  when  the  typical  signal  is  given. 
It  was  found  that  the  quantity  of  saliva  excreted  by  the 
signal  changes  in  a  definite  sense  and  quantity  when  the 
signal  varies  or  when  other  conditions  accompanying  the 
signal  vary. 

a  The  writer  is  indebted  for  the  details  of  Pawlow's  method  to  a  short 
review  by  Dr.  Morgulis.^  533 


MEMOBY  IMAGES  167 

Thus  in  one  dog  "by  persistent  training  a  conditioned 
reflex  has  been  established  to  the  stimulation  with  100 
oscillations  per  minute  of  the  metronome.  The  stimu 
lation  of  intermittent  sounds  of  such  frequency  called 
forth  6  to  10  drops  of  saliva  every  time.  The  interval 
between  successive  oscillations  was  then  modified,  the 
moment  of  the  disappearance  of  the  conditioned  salivary 
reflex  indicating  the  lowest  limit  of  differentiation.  With- 
out going  into  any  details  of  this  most  interesting  investi- 
gation or  quoting  actual  data,  I  will  say  that  the  dog 
could  sharply  distinguish  the  shortening  of  the  interval 
by  less  than  1/40  to  1/43  of  a  second.  Indeed  with  the 
well-developed  reflex  to  the  stimulation  of  100  beats  per 
minute  a  change  of  the  rate  to  either  96  or  104  beats  was 
immediately  reacted  upon  by  a  marked  diminution  or  even 
complete  cessation  of  the  flow  of  saliva." 

This  example  will  give  an  indication  how  sensitive  is 
this  method  of  measuring  the  effect  of  a  memory 
association. 

It  is  not  our  purpose  to  give  the  details  of  Pawlow's 
results — they  have  only  been  published  in  Russian  and 
are^  therefore  not  accessible  to  the  writer — but  to  show 
that  the  influence  of  an  associative  memory  image  is  as 
exactly  measurable  as,  e.g.,  the  direct  illumination  of  the 
eye;  and  moreover  that  what  we  call  a  memory  image 
is  not  a  " spiritual' '  but  a  physical  agency.  We  there- 
fore need  not  be  surprised  to  find  that  such  memory 
images  or  "conditioned  reflexes "  can  vary  and  multiply 
the  number  of  possible  tropistic  reactions. 

We  have  mentioned  in  the  previous  chapter  that  the 
stereotropism  in  the  mating  instinct  includes  apparently 
an  element  of  species  specificity  inasmuch  as  naturally 
only  males  and  females  of  the  same  species  mate.  The 


168  TEOPISMS 

late  Professor  Wliitman  has  shown  by  experiment  that 
this  specificity  is,  in  pigeons  at  least,  not  inherited  but 

effect  of  memory  images  (a  " conditioned  reflex "  in  the 
sense  of  Pawlow).  Whitman  took  the  eggs  or  young  of 
wild  species,  giving  them  to  the  domestic  ring-dove  to 
foster,  with  the  result,  that  the  young  reared  by  the  ring- 
doves ever  after  associated  with  ring-doves  and  tried  to 
mate  with  them.  Passenger  pigeons  when  reared  by  ring- 
doves refuse  to  mate  with  their  own  species  but  mate  with 
the  species  of  the  foster  parents.539  This  shows  inciden- 
tally that  racial  antagonism  is  not  inherited  but  acquired. 

We  have  mentioned  the  fact  that  the  mating  instinct 
is  determined  by  tropisms  aroused  by  specific  internal 
secretions,  and  that  in  isolated  male  pigeons  any  solid 
body  can  arouse  the  mating  reaction.  Craig540  raised 
male  pigeons  in  isolation  so  that  they  never  came  in  con- 
tact with  other  pigeons  until  they  were  adult.  One  pigeon 
was  hatched  in  July  and  isolated  in  August. 

Throughout  the  autumn  and  early  winter  this  bird  cooed  very  little. 
But  about  the  first  of  February  there  began  a  remarkable  development 
of  voice  and  social  behavior.  The  dove  was  kept  in  a  room  where  several 
men  were  at  work,  and  he  directed  his  display  behavior  toward  these 
men  just  as  if  they  belonged  to  his  own  species.  Each  time  I  put  food 
in  his  cage  he  became  greatly  excited,  charging  up  and  down  the  cage, 
bowing-and-cooing  to  me,  and  pecking  my  hand  whenever  it  came  within 
his  cage.  From  that  day  until  the  day  of  his  death,  Jack  continued  to 
react  in  this  social  manner  to  human  beings.  He  would  bow-and-coo  to 
me  at  a  distance,  or  to  my  face  when  near  the  cage ;  but  he  paid  greatest 
attention  to  the  hand — naturally  so,  because  it  was  the  only  part  with 
which  he  daily  came  into  direct  contact.  He  treated  the  hand  much  as 
if  it  were  a  living  bird.  Not  only  were  his  own  activities  directed  toward 
the  hand  as  if  it  were  a  bird,  but  he  received  treatment  by  the  hand  in 
the  same  spirit.  The  hand  could  stroke  him,  preen  his  neck,  even  pull 
the  feathers  sharply,  Jack  had  absolutely  no  fear,  but  ran  to  the  hand 
to  be  stroked  or  teased,  showing  the  joy  that  all  doves  show  in  the 
attentions  of  their  companions. 


MEMORY  IMAGES  169 

When  this  pigeon  was  almost  a  year  old  it  was  put  into 
a  cage  with  a  female  pigeon,  but  although  the  female 
aroused  the  sexual  instinct  of  the  formerly  isolated  male 
the  latter  did  not  mate  with  her,  but  mated  with  the 
hand  of  his  attendant  when  the  hand  was  put  into  the 
cage,  and  this  continued  throughout  the  season.  Thus  the 
memory  images  acquired  by  the  bird  at  an  impressionable 
age  and  period  perverted  its  sexual  tropisms. 

It  is  perhaps  of  more  importance  to  show  that  memory 
images  may  have  a  direct  orienting  influence.  The  chemo- 
tropic  phenomenon  of  an  insect  laying  its  egg  on  a  sub- 
stance which  serves  as  food  (for  both  mother  and  off- 
spring) and  for  which  the  mother  is  positively  chemo- 
tropic,  may  be  modified  by  an  act  of  associative  memory, 
e.g.,  when  a  solitary  wasp  drags  the  caterpillar  on  which 
it  lays  its  eggs  to  a  previously  prepared  hole  in  the  ground. 
The  essential  part  of  the  instinct,  the  laying  of  the  eggs 
on  the  caterpillar,  does,  perhaps,  not  differ  very  much 
from  the  fly  laying  its  eggs  on  decaying  meat;  and  the 
solitary  wasp  may  be  strongly  positively  chemotropic  for 
the  caterpillar  on  which  it  lays  the  eggs,  although  this 
has  not  yet  been  investigated.  But  the  phenomenon  is 
complicated  by  a  second  tropism,  which  we  will  call  the 
orienting  effect  of  the  memory  image.  As  is  well  known, 
the  wasp  before  "going  for"  the  caterpillar  digs  a  hole 
in  the  ground  to  which  it  afterwards  drags  the  caterpil- 
lar, often  from  a  distance.  The  finding  of  this  previously 
prepared  hole  by  the  returning  wasp,  the  writer  would 
designate  as  the  tropistic  or  orienting  effect  of  the  memory 
image  of  the  location  of  this  hole ;  meaning  thereby  that 
the  memory  image  of  the  location  of  this  hole  makes  the 
animal  return  to  this  location.  The  conduct  of  these  wasps 


170  TROPISMS 

is  familiar  to  many  readers  and  the  writer  may  be  par- 
doned for  quoting  from  a  formerly  published  observation. 

Ammophila,  a  solitary  wasp,  makes  a  small  hole  in  the  ground  and 
then  goes  out  to  hunt  for  a  caterpillar,  which,  when  found,  it  paralyses 
by  one  or  several  stings.  The  wasp  carries  the  caterpillar  back  to  the 
nest,  puts  it  into  the  hole,  and  covers  the  latter  with  sand.  Before  this 
is  done,  it  deposits  its  eggs  on  the  caterpillar  which  serves  the  young 
larva  as  food. 

An  Ammophila  had  made  a  hole  in  a  flower  bed  and  left  the  flower 
bed  flying.  A  little  later  I  saw  an  Ammophila  running  on  the  sidewalk 
of  the  street  in  front  of  the  garden,  dragging  a  caterpillar  which  it  held 
in  its  mouth.  The  weight  of  the  caterpillar  prevented  the  wasp  from 
flying.  The  garden  was  higher  than  the  sidewalk  and  separated  from  it 
by  a  stone  wall.  The  wasp  repeatedly  made  an  attempt  to  climb  upon 
the  stone  wall,  but  kept  falling  down.  .Suspecting  that  it  might  have 
a  hole  prepared  in  the  garden,  I  was  curious  to  see  whether  and  how 
it  would  find  the  hole.  It  followed  the  wall  until  it  reached  the  neigh- 
boring yard,  which  had  no  wall.  It  now  left  the  street  and  crawled 
into  this  yard,  dragging  the  caterpillar  along.  Then  crawling  through 
the  fence  which  separated  the  two  yards,  it  dropped  the  caterpillar  near 
the  foot  of  a  tree,  and  flew  away.  After  a  short  zigzag  flight  it  alighted 
on  a  flower  bed  in  which  I  noticed  two  small  holes.  It  soon  left  the  bed 
and  flew  back  to  the  tree,  not  in  a  straight  line  but  in  three  stages, 
stopping  twice  on  its  way.  At  the  third  stop  it  landed  at  the  place  where 
the  caterpillar  lay.  The  caterpillar  was  then  dragged  to  the  hole,  pulled 
into  it,  and  the  hole  was  covered  with  tiny  stones  in  the  usual  way.293 

It  is  not  enough  to  say  that  the  animal  possesses 
associative  memory  and  returns  to  the  hole;  we  must 
add  that  the  brain  image  of  the  region  of  the  hole  becomes 
the  source  of  a  forced  orientation  of  the  animal — of  an 
added  special  tropism — compelling  the  animal  to  return  to 
the  region  corresponding  to  the  image.  And  the  same 
may  be  said  in  regard  to  the  return  of  the  wasp  to  the 
caterpillar  which  had  been  temporarily  deposited  at  the 
foot  of  the  tree. 

This  example,  which  might  be  easily  multiplied,  will 


MEMORY  IMAGES  171 

show  the  addition  necessary  to  the  tropism  theory  to  make 
it  include  the  endless  number  of  reactions  in  which  associa- 
tive memory  is  involved.  The  psychiatrist  would  find  it 
easy  to  supply  numerous  examples  of  this  type  of  forced 
movements  toward  certain  objects  which  have  left  a 
memory  image.  Since  the  writer  has  not  investigated 
this  subject  sufficiently  he  is  not  in  a  position  to  give  more 
than  a  suggestion  for  the  direction  of  further  work.  He 
is  inclined  to  believe  that  with  this  enlargement  the  trop- 
ism theory  might  include  human  conduct  also  if  we  realize 
that  certain  memory  images  may  exercise  as  definite  an 
orienting  influence  as,  e.g.,  moving  retina  images  or  sex 
hormones. 

This  tentative  extension  of  the  forced  movement  or 
tropism  theory  of  animal  conduct  may  explain  why  higher 
animals  and  human  beings  seem  to  possess  freedom  of 
will,  although  all  movements  are  of  the  nature  of  forced 
movements.  The  tropistic  effects  of  memory  images  and 
the  modification  and  inhibition  of  tropisms  by  memory 
images  make  the  number  of  possible  reactions  so  great 
that  prediction  becomes  almost  impossible  and  it  is  this 
impossibility  chiefly  which  gives  rise  to  the  doctrine  of 
free  will.  The  theory  of  free  will  originated  and  is  held 
not  among  physicists  but  among  verbalists.  We  have 
shown  that  an  organism  goes  where  its  legs  carry  it  and 
that  the  direction  of  the  motion  is  forced  upon  the  organ- 
ism. When  the  orienting  force  is  obvious  to  us,  the  motion 
appears  as  being  willed  or  instinctive ;  the  latter  generally 
when  all  individuals  act  alike,  machine  fashion,  the  former 
when  different  individuals  act  differently.  When  a  swarm 
of  Daphnia  is  sensitized  with  C02  they  all  rush  to  the 
source  of  light.  This  is  a  machine-like  action,  and  many 


172  TBOPISMS 

will  be  willing  to  admit  that  it  is  a  forced  movement  or 
an  instinctive  reaction.  After  the  C02  has  evaporated 
the  animals  become  indifferent  to  light,  and  while  formerly 
they  had  only  one  degree  of  freedom  of  motion  they  now 
can  move  in  any  direction.  In  this  case  the  motions  appear 
to  be  spontaneous  or  free,  since  we  are  not  in  a  position  to 
state  why  Daphnia  a  moves  to  the  right  and  Daphnia  &, 
to  the  left,  etc.  As  a  matter  of  fact,  the  motion  of  each 
individual  is  again  determined  by  something  but  we  do 
not  know  what  it  is.  |  The  persistent  courtship  of  a  human 
male  for  a  definite  individual  female  may  appear  as  an 
example  of  persistent  will,  yet  it  is  a  complicated  tropism 
in  which  sex  hormones  and  definite  memory  images  are 
the  determining  factors.  Eemoval  of  the  sex  glands  abol- 
ishes the  courtship  and  replacing  the  sex  glands  of  an 
individual  by  those  of  the  opposite  sex  may  lead  to  a 
complete  reversal  of  the  sex  instincts.  (  What  appears  as 
persistent  will  action  is,  therefore,  essentially  a  tropistic 
reaction.  The  production  of  heliotropism  by  CO2  in 
Daphnia  and  the  production  of  the  definite  courtship  of 
the  male  A  for  the  female  B  are  similar  phenomena  differ- 
ing only  by  the  nature  of  the  hormones  and  the  additional 
tropistic  effects  of  certain  memory  images  in  the  case  of 
courtship.  Our  conception  of  the  existence  of  ' '  free  will ' ' 
in  human  beings  rests  on  the  fact  that  our  knowledge  is 
often  not  sufficiently  complete  to  account  for  the  orienting 
forces,  especially  when  we  carry  out  a  "premeditated" 
act,  or  when  we  carry  out  an  act  which  gives  us  pain  or 
may  lead  to  our  destruction,  and  our  incomplete  knowl- 
edge is  due  to  the  sheer  endless  number  of  possible  com- 
binations and  mutual  inhibitions  of  the  orienting  effect 
of  individual  memory  images. 


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49  BOHN,  G. :  Les  Convoluta  roscoffensis  et  la  theorie  des  causes  actu- 

elles.     Bull.  Mus.  Paris,  1903,  352-364. 
60  BOHN,  G. :  Theorie  nouvelle  du  phototropisme.    Compt.  rend.  Acad. 

Sc.,  1904,  cxxxix,  890-891. 

51  BOHN,  G. :  Attractions  et  oscillations  des  animaux  marms  sons  1'in- 

fluence  de  la  lumiere.  Recherches  nouvelles  relatives  an  phototac- 
tisme  et  au  phototropisme.  Mem.  Inst.  gener.  Psychol,  1905,  i, 
1-111. 

52  BOHN,  G. :  Impulsions  motrices  d'origine  oculaire  chez  les  crustaces. 

(Deuxieme  memoire  relatif  au  phototactisme  et  au  phototropisme.) 
Bull.  Inst.  gener.  Psychol,  1905,  v,  412-454. 

53  BOHN,  G. :  Intervention  des  reactions  oscillatoires  dans  les  tropismes. 

Compt.  rend.  Assoc.  Frangaise  avancement  des  Sc.,  Congres  de 
Reims,  1907,  700-706. 

54  BOHN,  G. :  Observations  biologiques  sur  le  branchellion  de  la  torpille. 

Bull.  Station  biol  Arcachon,  1907,  x,  283-296. 

55  BOHN,  G. :  Les  tropismes,  la  sensibilite  differentielle  et  les  associations 

chez  le  branchellion  de  la  torpille.  Compt.  rend.  Soc.  Biol.,  1907, 
Ixiii,  545-548. 

56  BOHN,  G. :  A  propos  des  lois  de  1'excitabilite  par  la  lumiere.     I.  Le 

retour  progressif  a  1'etat  d'immobilite,  apres  une  stimulation  mecan- 
ique.  Compt.  rend.  Soc.  Biol,  1907,  Ixiii,  655-658. 

57  BOHN,  G. :  II.  Du  changement  de  signe  du  phototropisme  en  tant  que 

manifestation  de  la  sensibilite  differentielle.  Compt.  rend.  Soc.  Biol., 
1907,  Ixiii,  756-759. 

68  BOHN,  G. :  Introduction  a  la  psychologie  des  animaux  a  symetrie 
rayonnee.  I.  Les  etats  physiologiques  des  actinies.  Bull.  Inst. 
gener.  Psychol.,  1907,  vii,  81-129 ;  135-182. 

59  BOHN,  G. :  II.  Les  essais  et  erreurs  chez  les  etoiles  de  mer  et  les  ophi- 

ures.    Bull.  Inst.  gener.  Psychol,  1908,  viii,  21-102. 

60  BOHN,  G. :  Les  rythmes  vitaux  chez  les  actinies.     Compt.  rend.  Assoc. 

Frangaise  avancement  des  Sc.,  1908,  613. 

61  BOHN,  G. :  De  1' orientation  chez  les  patelles.   Compt.  rend.  Acad.  Sc., 

1909,  cxlviii,  868-870. 

62  BOHN,  G. :  Les  variations  de  la  sensibilite  peripherique  chez  les  ani- 

maux.    Bull.  Sc.  France  et  Belgique,  1909,  xliii,  481-519. 

63  BOHN,  G. :  Quelques  problemes  generaux  relatif s  a  1'activite  des  ani- 

maux inferieurs.     Bull.  Inst.  gener.  Psychol,  1909,  ix,  439-466. 

64  BOHN,  G. :  Quelques  observations  sur  les  chenilles  des  dunes.    Bull. 

Inst.  gener.  Psychol,  1909,  ix,  543-549. 

65  BOHN,  G. :  La  naissanee  de  Pintelligence.    Paris,  1909. 


LITEEATURE  177 

66  BOHN,  G. :  Les  tropismes.     Rapport  VIme  Congr.  Internal.  Psychol. 

Geneve,  1909,  pp.  15. 

67  BOHN,  G. :  A  propos  les  lois  de  Pexcitabilite  par  la  lumiere.    III.  De 

Pinfluence  de  Peclairement  du  fond  sur  le  signe  des  reactions  vis-a- 
vis la  lumiere.    Compt.  rend.  Soc.  BioL,  1909,  Ixvi,  18-20. 

68  BOHN,  G. :  IV.  Sur  les  changements  periodiques  du  signe  des  reac- 

tions.   Compt.  rend.  Soc.  Blol.,  1909,  Ixvii,  4-6. 

69  BOHN,  G. :  V.  Intervention  de  la  vitesse  des  reactions  chimiques  dans 

la  desensibilisation  par  la  lumiere.     Compt.  rend.  Soc.  Biol.,  1910, 
Ixviii,  1114-1117. 

70  BOHN,  G. :  La  sensibilisation  et  la  desensibilisation  des  animaux. 

Compt.   rend.  Assoc.   Frangaise  avancement  des  Sc.,  Congres  de 
Toulouse,  1910,  214-222. 

71  BOHN,  G. :  Quelques  experiences  de  modification  des  reactions  chez  les 

animaux,  suivies  de  considerations  sur  les  meoanismes  chimiques 
de  revolution.    Bull  Sc.  France  et  Belgique,  1911,  xlv,  217-238. 
7/2  BOHN,  G. :  La  nouvelle  psychologic  animale.    Paris,  1911,  pp.  200. 

73  BOHN,  G. :  La  sensibilite  des  animaux  aux  variations  de  pression. 

Compt.  rend.  Acad.  Sc.,  1912,  cliv,  240-242. 

74  BOHN,  G. :  Les  variations  de  la  sensibilite  en  relation  avec  les  varia- 

tions de  Petat  chimique  interne.     Compt.  rend.  Acad.  Sc.,  1912, 
cliv,  388-391. 

75  BOHN,  G. :  L'etude  des  phenomenes  mnemiques  chez  les  organismes 

inferieurs.    J.  Psychol.  u.  NeuroL,  1913,  xx,  199-209. 

76  BORING,  E.  G. :  Note  on  the  Negative  Reaction  Under  Light  Adapta- 

tion in  the  Planarian.    J.  Animal  Behav.,  1912,  ii,  229-248. 

77  BORN,   G. :   Biologische  Untersuchungen.    Ueber  den   Einfluss   der 

Schwere  auf  das  Froschei.    Arch.  mikr.  Anat.,  1885,  xxiv,  475. 

78  BREUER,  J. :  Ueber  die  Funktion  der  Bogengange  des  Ohrlabyrinths. 

Med.  Jahrb.,  1874. 

79  BREUER,  J. :  Beitrage  zur  Lehre  vom  statischen  S'inne.    Med.  Jahrb., 

1875. 

80  BREUER,  J. :  Ueber  die  Funktion  der  Otolithenapparate.     Arch.  ges. 

PhysioL,  1891,  xlviii,  195-306. 
80a  BREUER,    J. :    Ueber    den    Galvanotropismus    (Galvanotaxis)    bei 

Fischen.     Sitzngsb.  Akad.  Wiss.  Wien.  mathem.-naturw.  Kl.,  1905, 

cxiv,  27-56. 
806  BREUER,  J.,  and  KREIDL,  A. :  Ueber  die  scheinbare  Drehung  des 

Gesichtsfeldes,   wahrend   der   Einwirkung   einer   Centrifugalkraft. 

Arch.  ges.  PhysioL,  1898,  Ixx,  494-510. 

81  BRUCHMANN,    H. :    Chemotaxis    der    Lycopodium-Spermatozoiden, 

Flora,  1908-09,  xcix,  193-202. 
12 


178  TEOPISMS 

82  BRUN,  E. :  Die  Raumorientierung  der  Ameisen  und  das  Orientierungs- 

problem  im  allgemeinen.    Jena,  1914,  pp.  242. 

83  BRUNDIN,  T.  M. :  Light  Reactions  of  Terrestrial  Amphipods.    J.  Ani- 

mal Behav.,  1913,  iii,  334-352. 

84  v.  BUDDENBROCK,  W. :  Untersuchungen  iiber  die  Schwimmbewegungen 

und  die  Statocysten  der  G-attung  Pecten.     Sitzngsb.  Heidelberger 
Akad.  Wiss.j  mathem.-naturw.  Kl.,  1911,  pp.  24. 

85  v.  BUDDENBROCK,  W. :  Ueber  die  Funktion  der  Statocysten  im  Sande 

grabender   Meerestiere    (Arenicola   und   Synapta}.    Biol.    Centr., 
1912,  xxxii,  564-585. 

86  v.   BUDDENBROCK,  W. :   Ueber  die  Funktion   der  Statocysten  von 

Branchiomma   vesiculosum.     Verhandl.    naturhist.-med.     Vereines, 
Heidelberg,  1913,  N.F.  xii,  256-261. 

87  v.  BUDDENBROCK,  W. :  Ueber  die  Orientierung  der  Krebse  im  Raum. 

Zool.  Jahrb.  Abt.  Zool.,  1914,  xxxiv,  479-514. 

88  v.  BUDDENBROCK,  W. :  A  Criticism  of  the  Tropism  Theory  of  Jacques 

Loeb.    J.  Animal  Behav.,  1916,  vi,  341-366. 

89  BULLER,  A.  H.  R. :  Contributions  to  Our  Knowledge  of  the  Physiology 

of  the  Spermatozoa  of  Ferns.    Annals  Bot.,  1900,  xiv,  543-582. 

90  BULLER,  A.  H.  R. :  Is  Chemotaxis  a  Factor  in  the  Fertilization  of  the 

Eggs  of  Animals?    Quart.  J.  Micr.  Sc.,  1902-03,  xlvi,  145-176. 

91  BOYSEN-JENSEN,  P. :  Ueber  die  Leitung  des  phototropischen  Reizes  in 

der  J.i;ewakoleoptile.     Ber.  bot.  Ges.,  1913,  xxxi,  559-566. 

92  BUNTING,  M. :  Ueber  die  Bedeutung  der  Otolithenorgane  f  iir  die  geo- 

tropischen  Funktionen  von  Astacus  fluviatilis.    Arch.  ges.  Physiol., 
1893,  liv,  531-537. 

93  CARLGREN,  0. :  Der  Galvanotropismus  und  die  innere  Kataphorese. 

Z.  allg.  Physiol.,  1905,  v,  123-130. 

94  CARLGREN,  0. :  Ueber  die  Einwirkung  des  konstanten  galvanischen 

Stromes  auf  niedere  Organismen.    Arch.  Anat.  u.  Physiol.,  Physiol. 
Abt.,  1900,  49-76. 

95  CARPENTER,  F.  W. :  The  Reactions  of  the  Pomace  Fly  (Drosophila 

ampelophila,  Loew)  to  Light,  Gravity,  and  Mechanical  Stimulation. 
Am.  Nat.,  1905,  xxxix,  157-171. 

95o  CLAPAREDE,  E. :  Les  tropismes  devant  la  psychologic.    J.  Psychol.  u. 
Neurol.,  1908,  xiii,  150-160. 

96  CLARK,  G.  P. :  On  the  Relation  of  the  Otocysts  to  Equilibrium  Phe- 

nomena  in    GelasimiAs   pugilator  and  Platyonichus   ocellatus.     J. 
Physiol.,  1896,  xix,  327-343. 

97  CLARK,  0.  L. :  Ueber  negativen  Phototropisnms  bei  Avena  sativa. 

Z.  Bot.,  1913,  v,  737-770. 


LITEBATURE  179 

98  COEHN,  A.,  and  BARRATT,  W. :  Ueber  Galvanotaxis  vom  Standpunkte 

der  physikalischen  Chemie.     Z.  allg.  Physiol.,  1905,  v,  1-9. 

99  COHN,  F. :  Ueber  die  Gesetze  derBewegung  mikroskopischer  Tiere  und 

Pflanzen  unter  Einfluss  des  Lichtes.    Jahr.-ber.  Schles.  Ges.  vaterl. 
Kultur,  1864,  xlii,  35-36. 

100  COLE,  L.  J. :  The  Influence  of  Direction  vs.  Intensity  of  Light  in  De- 

termining the  Phototropic  Responses  of  Organisms.     Science,  1907, 
xxv,  784. 

101  CONGDON,  E.  D. :  Recent  Studies  Upon  the  Locomotor  Responses  of 

Animals  to  White  Light.     /.  Comp.  Neurol.  and  Psychol.,  1908, 
xviii,  309-328. 

102  CORNETZ,  V. :  Ueber  den  Gebrauch  des  Ausdruckes  "  tropisch  "  und 

liber  den  Charakter  der  Richtungskraft  bei  Ameisen.     Arch.  ges. 

Physiol,  1912,  cxlvii,  215-233. 
i°3  COWLES,  R.  P. :  Stimuli  Produced  by  Light  and  by  Contact  with  Solid 

Walls  as  Factors  in  the  Behavior  of  Ophiuroids.     /.  Exp.  Zool., 

1910,  ix,  387-416. 
i°4  COWLES,  R.  P. :  Reaction  to  Light  and  Other  Points  in  the  Behavior  of 

the  Starfish.     Papers  from  Tortugas  Lab.  Carnegie  Inst.   Wash- 
ington, 1911,  iii,  95-110. 
K>5  COWLES,  R.  P. :  The  Influence  of  White  and  Black  Walls  on  the 

Direction  of  Locomotion  of  the  Starfish.    J.  Animal  Behav.,  1914, 

iv,  380-382. 
1050  CRAIG,  W. :  The  Voices  of  Pigeons  Regarded  as  a  Means  of  Social 

Control.    Am.  J.  Sociology,  1908,  xiv,  86-100. 
1056  CRAIG,  W. :  Male  Doves  Reared  in  Isolation.  J.  Animal  Behav.,  1914, 

iv,  121-133. 
105c  CRAIG,  W. :  Appetites  and  Aversions  as  Constituents  of  Instincts. 

Biol.  Bull.,  1918,  xxxiv,  97-107. 

106  CROZIEJI,  W.  J. :  The  Orientation  of  a  Holothurian  by  Light.     Am. 

J.  Physiol,  1914,  xxxvi,  8-20. 

107  CROZIER,  W.  J. :  The  Behavior  of  Holothurians  in  Balanced  Illumina- 

tion.   Am.  J.  Physiol.,  1917,  xliii,  510-513. 

108  CROZIER,  W.  J. :  The  Photoreceptors  of  Amphioxus.    Anat.  Eec.,  1917, 

xi,  520. 

1080  CROZIER,  W.  J.:  The  Photic  Sensitivity  of  Balanoglossus.   J.  Exp. 
Zool.,  1917,  xxiv,  211-217. 

109  CZAPEK,  F. :  Ueber  Zusammenwirken  von  Heliotropismus  und  Geo- 

tropismus.     Sitzngsb.  Akad.  Wiss.  Wien.  mathem.-naturw.  KL,  1895, 
civ. 

110  CZAPEK,  F. :  Untersuchungen  iiber  Geotropismus.    Jahrb.  wiss.  Bot., 

1895,  xxvii,  243-339. 


180  TROPISMS 

111  CZAPEK,  F. :  Weitere  Beitrage  zur  Kenntnis  der  geotropischen  Reiz- 

bewegungen.    Jahrb.  uriss.  Bot.,  1898,  xxxii,  175-308. 

112  DALE,  H.  H. :  Galvanotaxis  and  Chemotaxis  of  Ciliate  Infusoria. 

J.  Physiol,  1901,  xxvi,  291-361. 

113  DAVENPORT,  C.  B. :  Experimental  Morphology.     Part  I.    Effects  of 

Chemical  and  Physical  Agents  Upon  Protoplasm.     New  York,  1897. 

114  DAVENPORT,  C.  B.,  and  CANNON,  W.  B. :  On  the  Determination  of  the 

Direction  and  Rate  of  Movement  of  Organisms  by  Light.  J.  Phys- 
iol, 1897,  xxi,  22-32. 

1 15  DAVENPORT,  C.  B.,  and  LEWIS,  F.  T. :  Phototaxis  of  Daphnia.  Science, 

1899,  ix,  368. 

116  DAVENPORT,  C.  B.,  and  PERKINS,  H. :  A  Contribution  to  the  Study  of 

Geotaxis  in  the  Higher  Animals.     /.  Physiol,  1897,  xxii,  99-110. 
"7  DAY,  E.  C.:  The  Effect  of  Colored  Light  on  Pigment  Migration  in 
the  Eye  of  the  Crayfish.    Bull  Mus.  Comp.  Zool,  1911,  liii,  303- 
343. 

118  DELAGE,  Y. :  Etude  experimentale  sur  les  illusions  statiques  et  dyna- 

miques  de  direction  pour  servir  a  determiner  les  fonctions  des 
eanaux  semicirculaires  de  Foreille  interne.  Arch.  Zool  exper.  et 
gener.,  1886,  (2)  iv. 

119  DELAGE,  Y. :  Sur  une  fonction  nouvelle  des  otocystes  comme  organcs 

d'orientation  locomotrioe.  Arch.  Zool.  exper.  et  gener. ,  1887,  (2) 
v,  1-26. 

1 20  DEWITZ,  J. :  Ueber  die  Vereinigung  der  Spermatozoen  mit  dem  Ei. 

Arch.  ges.  Physiol,  1885,  xxxvii,  219-223. 

121  DEWITZ,   J. :   Ueber   Gesetzinassigkeit  in   der   Ortsveranderung  der 

Spermatozoen  und  in  der  Vereinigung  derselben  mit  dem  Ei.  Arch, 
ges.  Physiol,  1886,  xxxviii,  358-385. 

122  DEWITZ,  J. :  Ueber  den  Rheotropismus  bei  Tieren.     Arch.  Physiol, 

1899  (Suppl.),  231-244. 

1 23  DOLLEY,  W.  L.,  JR.  :  Reactions  to  Light  in  Vanessa  antiopa,  with 

Special  Reference  to  Circus  Movements.  J.  Exp.  Zool,  1916,  xx, 
357-420. 

124  DRIESCH,  H. :  Heliotropismus  bei  Hydroidpolypen.     Zool  Jahrb., 

1890,  v,  147-156. 

125  DRIESCH,  H. :  Die  taktische  Reizbarkeit  der  Mesenchymzellen  von 

Echinus  microtuberculatus.  Arch.  Entwcklngsmech.,  1896,  iii,  362- 
380. 

126  DRIESCH,  H. :  Die  organischen  Regulationen.    Leipzig,  1901,  pp.  228. 

1 27  DUBOIS,  R. :    Sur  le  mecanisme   des  fonctions   photodermatique  et 

photogenique  dans  le  siphon  du  Pholas  dactylus.  Compt.  rend. 
Acad.  Sc.,  1889,  cix,  233-235. 


LITEEATURE  181 

1 28  DUBOIS,  R. :  Sur  Faction  des  agents  modificateurs  de  la  contraction 

photodermatique  chez  le  Pholas  dactylus.     Compt.  rend.  Acad.  Sc., 

1898,  cix,  320-322. 
!29  DUBOIS,  R. :  Sur  la  perception  des  radiations  lumineuses  par  la  peau, 

chez  les  Protees  aveugles  des  grottes  de  la  Carniole.    Compt.  rend. 

Acad.  Sc.,  1890,  ex,  358-361. 
1 3°  DUBOIS,  R. :  Note  sur  1'action  de  la  lumiere  sur  les  echinodermes 

(oursin).      Commun.   9me.   Cong,  internat.   Zool.,  Monaco,   1913, 

(1),  8-9. 

131  DUSTIN,  A.  P.:  Le  role  des  tropismes  et  de  I'odogenese  dans  la  re- 

generation du  systeme  nerveux.    Arch.  BioL,  1910,  xxv,  269-388. 

132  ENGELMANN,  T.  W. :  Ueber  Reizung  kontraktilen  Protoplasmas  durch 

plotzliche  Beleuchtung.     Arch.  ges.  Physiol,  1879,  xix,  1-7. 

133  ENGELMANN,  T.  W. :  Ueber  Licht-  und  Farbenperzeption  niederster 

Organismen.     Arch.  ges.  Physiol.,  1882,  xxix,  387-400. 

134  ENGELMANN,  T.  W. :  Bacterium  photometricum.    Ein  Beitrag  zur  ver- 

gleichenden  Physiologie  des  Licht-  und  Farbensinnes.     Arch.  ges. 
Physiol,  1882,  xxx,  95-124. 

13s  ENGELMANN,  T.  W.:  Ueber  die  Funktion  der  Otolithen.  Zool.  Anz., 
1887,  x,  591,  664. 

1 36  ENGELMANN,  T.  W. :  Die  Purpurbakterien  und  ihre  Beziehungen  zum 

Licht.    Bot.  Ztg.,  1888,  xlvi,  661-669,  677-689,  693-701,  709-720. 

137  ENGLISCH,  E. :  Ueber  die  Wirkung  intermittierender  Belichtungen 

auf  Bromsilbergelatine.     Arch.  wiss.  Phot.,  1899,  i,  117-131. 

138  ENGLISCH,  E. :  Ueber  den  zeitlichen  Verlauf  der  durch  das  Licht 

verursachten  Veranderungen  der  Bromsilbergelatine.     Arch.  wiss. 
Phot.,  1900,  ii,  131-134. 

139  ERHARD,  H. :  Beitrag  zur  Kenntnis  des  Lichtsinnes  der  Daphniden. 

BioL  Centr.,  1913,  xxxiii,  494-496. 

140  ESTERLY,  C.  O. :  The  Reactions  of  Cyclops  to  Light  and  Gravity. 

Am.  J.  Physiol,  1907,  xviii,  47-57. 

141  EWALD,  J.  R. :  Physiologische  Untersuchungen  iiber  das  Endorgan 

des  Nervus  octavus.     Wiesbaden,  1892. 

14  2  EWALD,  J.  R. :  Ueber  die  Wirkung  des  galyanischen  Stroms  bei  der 
Langsdurchstromung  ganzer  Wirbeltiere.  Arch.  ges.  Physiol.,  1894, 
Iv,  606-621  (Berightigung,  1894,  Ivi,  354). 

143  EWALD,  W.  E. :  Ueber  Orientierung,  Lokomotion  und  Lichtreaktionen 

einiger  Cladoceren  und  deren  Bedeutung  fur  die  Theorie  der  Trop- 
ismen.     BioL  Centr.,  1910,  xxx,  1-16,  49-63,  379-399. 

144  EWALD,  W.  E. :  On  Artificial  Modification  of  Light  Reactions  and  the 

Influence  of  Electrolytes  on  Phototaxis.     J.  Exp.  Zool,  1912,  xiii, 
591-612. 


182  TEOPISMS 

145  EWALD,  W.  E. :  The  Applicability  of  the  Photochemical  Energy  Law 

to  Light  Reactions  in  Animals.     Science,  1913,  xxxviii,  236-237. 

146  EWALD,  W.  E. :  1st  die  Lehre  vom  tierischen  Phototropismus  wider- 

legt?    Arch.  Entwcklngsmech.,  1913,  xxxvii,  581-598. 

147  EWALD,  W.  E. :  Versuche  zur  Analyse  der  Licht-  und  Farbenreak- 

tionen  eines  Wirbellosen  (Daphnia  pulex) .  Z.  Sinnesphysiol,  1914, 
xlviii,  285-324. 

148  EYCLESHYMER,  A.  C. :  The  Reactions  to  Light  of  the  Decapitated 

Young  Necturus.  J.  Comp.  Neurol.  and  Psychol.,  1908,  xviii,  SOS- 
SOS. 

149  FAUVEL,  P.,  and  BOHN,  G. :  Le  rythme  des  marees  chez  les  diatomees 

littorales.    Compt.  rend.  Soc.  BioL,  1907,  Ixii,  121-123. 

150  FIGDOR,  W. :  Ueber  Helio-  und  Geotropismus  der  Gramineenblatter. 

Per.  bot.  Ges.,  1905,  xxiii,  182-191. 

151  FIGDOR,  W. :  Experimentelle  Studien  iiber  die  heliotropische  Empfind- 

lichkeit  der  Pflanzen.     Wiesner  Festschrift,  Wien,  1908. 

152  FIGDOR,  W. :  Heliotropische  Reizleitung  bei  Begonia-Slattern.    Ann. 

Jardin  bot.  Buitenzorg.,  1910  (Suppl.),  iii,  453-460. 

153  FIGDOR,  W. :  Ueber  thigmotropische  Empfindlichkeit  der  Asparagus- 

Sprosse.  Sitzngsb.  Akad.  Wiss.  Wien.  mathem.-naturw.  Kl.  Abt. 
I,  1915,  cxxiv,  353. 

154  FITTING,  H. :  Untersuchungen  iiber  den  geotropischen  Reizvorgang. 

Jahrb.  wiss.  Bot.,  1905,  xli,  221-398. 

155  FLOURENS,  P. :  Recherches  experimentales  sur  les  proprietes  et  les 

fonctions  du  systeme  nerveux  dans  les  animaux  vertebres.  Paris, 
1842,  pp.  xxviii  +  516. 

156  FORSSMAN,  J. :  Ueber  die  Ursaehen,  welche  die  Wachstumsrichtung 

der  peripheren  Nervenfasern  bei  der  Regeneration  bestimmen. 
Beitr.  path.  Anat.,  1898,  xxiv,  56-100. 

157  FORSSMAN,  J. :  Zur  Kenntnis  des  Neurotropismus.    Beitr.  path.  Anat., 

1900,  xxvii,  407-430. 

158  FRANDSEN,  P. :  Studies  on  the  Reactions  of  Limax  maximus  to  Direc- 

tive Stimuli.     Proc.  Am.  Acad.  Arts  and  Sc.,  1901,  xxxvii,  185-227. 

159  FRANZ,  V. :  Phototaxis  und  Wanderung.    Nach  Versuchen  mit  Jung- 

fischen  und  Fischlarven.  Int.  Eev.  ges.  Hydrobiol.  u.  Hydro  graphic, 
1910,  iii,  306-334. 

16<>  FRANZ,  V.:  Beitrage  zur  Kenntnis  der  Phototaxis.  Nach  Versuchen 
an  Siisswassertieren.  Int.  Eev.  ges.  Hydrobiol.  u.  Hydrographie, 
Biol.  Suppl.  (2),  1911, 1-11. 

161  FRANZ,  V.:  Weitere  Phototaxisstudien.  I.  Zur  Phototaxis  bei 
Fischen.  II.  Phototaxis  bei  marinen  Crustaceen.  III.  Phototak- 
tische  Lokomotionsperioden  bei  Hemimysis.  Int.  Eev.  ges.  Hydro- 
biol. u.  Hydrographie,  Biol.  Suppl.  (3),  1911,  1-23. 


LITERATURE  183 

162  FRANZ,  V. :  Zur  Frage  der  vertikalen  Wanderimgen  der  Planktontiere. 

Arch.  Hydrobiol.  u.  Planktonkunde,  1912,  vii,  493-499. 

163  FRANZ,  V. :  Die  phototaktischen  Erscheintmgen  im  Tierreiche  und 

ihre  Rolle  im  Freileben  der  Tiere.    Zool.  Jahrb.,  1913,  xxxiii,  259- 
286. 

164  v.  FRISCH,  K. :  Ueber  farbige  Anpassung  bei  Fischen.    Zool.  Jahrb., 

1912,  xxxii,  171-230. 

165  v.  FRISCH,  K.:  Sind  die  Fische  farbenblind?    Zool.  Jahrb.,  1912, 

xxxiii,  107-126. 

166  v.  FRISCH,  K. :  Ueber  die  Farbenanpassung  des  Crenilabrus.    Zool. 

Jahrb.,  1912,  xxxiii,  151-164. 

i67v.  FRISCH,  K. :  Weitere  Untersuchungen  iiber  den  Farbensinn  der 
Fische.    Zool.  Jahrb.,  1913,  xxxiv,  43-68. 

168  v.  FRISCH,  K. :  Der  Farbensinn  und  Formensinn  der  Biene.    Zool. 

Jahrb.,  1914,  xxxv,  1-182. 

169  v.  FRISCH,  K.,  and  KUPELWIESER,  II.:  Ueber  den  Einflnss  der  Licht- 

farbe  auf  die  phototaktischen  Reaktionen  niederer  Krebse.     Biol. 
Centr.,  1913,  xxxiii,  517-552. 

ITO  FROHLICH,  F.  W. :  Vergleichende  Untersuchungen  liber  den  Licht- 
und  Farbensinn.     Deutsch.  med.  Wchnschr.,  1913,  xxxix,  1453-1456. 

171  FROSCHEL,  P. :  Untersuehung  iiber  die  heliotropische  Prasentations- 

zeit.     I.  Sitzngsb.  Akad.  Wiss.  Wien.  mathem.-naturw.  Kl.,  1908, 
cxvii,  235-256. 

172  FROSCHEL,  P.:  Untersuchung  iiber  die  heliotropische  Prasentations- 

zeit.     II.  Sitzngsb.  Akad.  Wiss.  Wien.  mathem.-naturw.  Kl.,  1909, 
cxviii,  1247-1294. 

173  FUCHS,  R.  F. :  Der  Farbenwechsel  und  die  chromatische  Hautf  unk- 

tion   der  Tiere.     Winterstein's  Handb.   vergl.   Physiol.,   1914,   iii, 
I.  Halfte  2,  1189-1656. 

1™  GALIANO,  E.  F. :  Beitrag  zur  Untersuchung  der  Chemotaxis  der  Para- 
m&cien.  Z.  allg.  Physiol.,  1914,  xvi,  359-372. 

175  GARRET,  W.  E.:  The  Effect  of  Ions  Upon  the  Aggregation  of  Flagel- 

lated Infusoria.    Am.  J.  Physiol.,  1900,  iii,  291-315. 

176  GARREY,  W.  E. :  A  Sight  Reflex  Shown  by  Sticklebacks.    Biol.  Bull, 

1905,  viii,  79-84. 

177  GARREY,  W.  E. :  Proof  of  the  Muscle  Tension  Theory  of  Heliotropism. 

Proc.  Nat.  Acad.  Sc.,  1917,  iii,  602-609. 

178  GOLTZ,  F. :  Ueber  die  Verrichtungen  des  Grosshirns.    I-V.  Arch.  ges. 

Physiol,  1876,  xiii,  1-44;  1877,  xiv,  412-443;  1879,  xx,  1-54;  1881, 
xxvi,  1-49;  1884,  xxxiv,  450-505. 

179  GRABER,  V.:  Fundamental versuche  iiber  die  Helligkeits-  und  Farben- 

empfindlichkeit  augenloser  und  geblendeter  Tiere.    Sitzngsb.  Akad. 
Wiss.  Wien,  1883,  Ixxxvii,  201-236. 


184  TEOPISMS 

180  GRABER,  V. :  Grundlinien  zur  Erforschung  des  Helligkeits-  und  Far- 

bensinnes  der  Tiere.     Leipzig,  1884,  pp.  vii  +322. 

181  GRABER,  V. :  Ueber  die  Helligkeits-  und  Farbenempfindlichkeit  einiger 

Meertiere.    Sitzngsb.  Akad.  Wiss.  Wien.,  1885,  xci. 

182  GRABER,  V.:  Thermische  Experimente  an  der  Kiichenschabe  (Peri- 

planeta  orientalis).     Arch.  ges.  Physiol,  1887,  xli,  240-256. 

183  GROOM,  T.  T.,  and  LOEB,  J. :  Der  Heliotropismus  der  Nauplien  von 

Balanus  perforatus  und  die  periodischen  Tiefenwanderungen  pelag- 
ischer  Tiere.     Biol.  Centr.,  1890,  x,  160-177. 

184  GROSS,  A.  0. :  The  Reactions  of  Arthropods  to  Monochromatic  Lights 

of  Equal  Intensities.     J.  Exp.  ZooL,  1913,  xiv,  467-514. 

185  HABERLANDT,  G. :   Ueber  die  Perzeption  des  geotropischen  Reizes. 

Ber.  bot.  Ges.,  1900,  xviii,  261-272. 

186  HADLEY,  P.  B.:  The  Relation  of  Optical  Stimuli  to  Rheotaxis  in  the 
.American  Lobster  (Homarus  americanus>) .     Am.  J.  Physiol,  1906, 

xvii,  326-343. 

187  HADLEY,  P.  B. :   Galvanotaxis  in  Larva?  of  the  American  Lobster 

(Homarus  americanus).     Am.  J.  Physiol.,  1907,  xix,  39—52. 

188  HADLEY,  P.  B. :  The  Reaction  of  Blinded  Lobsters  to  Light.     Am.  J. 

Physiol.,  1908,  xxi,  180-199. 

189  HADLEY,  P.  B. :  Reactions  of  Young  Lobsters  Determined  by  Food 

Stimuli.    Science,  1912,  xxxv,  1000-1002. 

190  HARPER,  E.  H. :  Reactions  to  Light  and  Mechanical  Stimuli  in  the 

Earthworm,  Perichata  bermudensis  (Beddard).     Biol.  Bull,  1905, 
x,  17-34. 

101  HARPER,  E.  H. :  Tropic  and  Shock  Reactions  in  Perichata  and  Lum- 
bricus.    J.  Comp.  Neurol.  and  Psychol.,  1909,  xix,  569-587. 

192  HARPER,  E.  H. :  The  Geotropism  of  Paramcecium.    J.  Morphol.,  1911, 

xxii,  993-1000. 

193  HARPER,  E.  H. :  Magnetic  Control  of  Geotropism  in  Paramcecium. 

J.  Animal  Behav.,  1912,  ii,  181-189. 

194  HARRINGTON,  N".  R.,  and  LEAMING,  E. :  The  Reaction  of  Amoeba  to 

Lights  of  Different  Colors.     Am.  J.  Physiol,  1899,  iii,  9-18. 

195  HASEMAN,  J.  D. :  The  Rhythmical  Movements  of  Littorina  littorea 

Synchronous  with  Ocean  Tides.     Biol.  Bull,  1911,  xxi,  113-121. 

196  HAUSMANN,  W. :  Die  photodynamische  Wirkung  des  Chlorophylls 

und  ihre  Beziehung  zur  photosynthetischen  Assimilation  der  Pflan- 
zen.    Jahrb.  wiss.  Bot.,  1909,  xlvi,  599-623. 

197  HELMHOLTZ,  H. :  Handbuch  der  physiologischen  Optik.    Hamburg, 

1909-11,  3.  Ed. 

198  HENRI,  MME.  V.,  and  HENRI,  V. :  Excitation  des  organismes  par  les 

rayons  ultra-violets.     Compt.  rend.  Soc.  Biol,  1912,  Ixxii,  992-996; 
Ixxiii,  326-327. 


LITERATURE  185 

1<JU  HENRI,  V.,  and  LARGUIER  DES  BANCELS,  J. :  Photochimie  de  la  retine. 
J.  Physiol.  et  Path,  gener.,  1911,  xiii,  841-856. 

200  HENRI,  V.,  and  LARGUIER  DES  BANCELS,  J. :  Un  nouveau  type  de  temps 

de  reaction.     Compt.  rend.  Soc.  BioL,  1912,  Ixxiii,  55-56. 

201  HENRI,  V.,  and  LARGUIER  DES  BANCELS,  J. :  L'excitation  provoquee 

par  les  rayons  ultra-violets  comparee  avec  les  excitations  visuelle  et 
nerveuse,  d'une  part,  et  les  reactions  photochimiques,  d'autre.  Lois 
des  phenomenes.  Compt.  rend.  Soc.  Biol.,  1912,  Ixxiii,  328-329. 

202  HENRI,  V.,  and  LARGUIER  DES  BANCELS,  J. :  S'ur  Interpretation  des 

lois  de  Weber  et  de  Jost :  reeherches  sur  les  reactions  des  Cyclops  ex- 
posees  a  la  lumiere  ultra-violette.  Arch.  Psychol.,  1912,  xii,  329-342. 

203  HERBST,  C. :  Ueber  die  Bedeutung  der  Reizphysiologie  fur  die  kausale 

Auffassung  von  Vorgangen  in  der  tierischen  Ontogenese.  I.  Biol. 
Centr.,  1894,  xiv,  657-666,  689-697,  727-744,  753-771,  800-310. 

204  HERMANN,  L. :  Eine  Wirkung  galvanischer  Strome  auf  Organismen. 

Arch.  ges.  Physiol.,  1885,  xxxvii,  457-460. 

205  HERMANN,   L. :   Weitere  Untersuchungen   iiber  das   Verhalten   der 

Froschlarven  im  galvanischen  Strome.  Arch.  ges.  Physiol.,  1886, 
xxxix,  414-419. 

20  5 a  HERMANN,  L.,  and  MATTHIAS,  F. :  Der  Galvanotropismus  der  Larven 
von  Eana  temporaria  und  der  Fische.  Arch.  ges.  Physiol.,  1894, 
Ivii,  391-405. 

206  HERMS,  W.  B. :  The  Photic  Reactions  of  Sacrophagid  Flies,  Espe- 

cially Lucilia  caesar  Linn,  and  Calliphora  vormitoria  Linn.  J.  Exp. 
Zool.,  1911,  x,  167-226. 

207  HERTEL,  E. :  Ueber  die  Beeinflussung  des  Organismus  durch  Licht, 

speciell  durch  die  chemisch  wirksamen  Strahlen.  Z.  allg.  Physiol. , 
1904,  iv,  1-43. 

208  HERTEL,  E. :  Ueber  physiologische  Wirkung  von  Strahlen  verschie- 

dener  Wellenlange.     Z.  allg.  Physiol.,  1905,  v,  95-122. 

209  HESS,  C. :  Untersuchungen  iiber  den  Lichtsinn  bei  Fischen.     Arch. 

Augenheilk.,  1909,  Ixiv,  1-38. 

210  HESS,  C. :  Untersuchungen  tiber  den  Lichtsinn  bei  wirbellosen  Tieren. 

Arch.  Augenheilk.,  1909,  Ixiv,  39-61. 

211  HESS,  C. :  Neue  Untersuehungen  iiber  den  Lichtsinn  bei  wirbellosen 

Tieren.     Arch.  ges.  Physiol.,  1910,  cxxxvi,  282-367. 

212  HESS,  C. :  Experimentelle  Untersuchungen  zur  vergleichenden  Physi- 

ologic des  Gesichtssinnes.     Arch.  ges.  Physiol.,  1911,  cxlii,  405-446. 

213  HESS,  C. :  Der  Gesichtssinn.     Winterstein's  Handb.  vergl.  Physiol., 

1912,  iv,  555-840. 

214  HESS,  C. :  Neue  Untersuchungen  zur  vergleichenden  Physiologic  des 

Gesichtsinnes.     Zool.  Jahrb.  Abt.  Zool.,  1913,  xxxiii,  387-440. 


186  TKOPISMS 

215  HESS,  C. :  Experimentelle  Untersuchungen  uber  den  angeblichen  Far- 

bensinn  der  Bienen.     Zool.  Jahrb.  Abt.  Zool,  1913,  xxxiv,  81-106. 

216  HESS,  C. :  Eine  neue  Methode  zur  Untersuchung  des  Lichtsinnes  bei 

Krebsen.    Arch,  vergl  Ophthalmol,  1913-14,  iv,  52-67. 

217  HESS,  C. :  Untersuchungen  iiber  den  Liehtsinn  mariner  Wiirmer  und 

Krebse.     Arch.  ges.  Physiol,  1914,  civ,  421-435. 

218  HE^S,   C. :  Untersuchungen  fiber  den  Lichtsinn  bei  Echinodermen. 

Arch.  ges.  Physiol.,  1914,  clx,  1-26. 

219  HESS,  C. :  Messende  Untersuchung  des  Lichtsinnes  der  Biene.     Arch. 

ges.  Physiol,  1916,  clxiii,  289-320. 

220  HESSE,  R. :  Untersuchungen  iiber  die  Organe  der  Lichtempfindung  bei 

niederen  Tieren.  I.  Die  Organe  der  Lichtempfindung  bei  den  Lubri- 
ciden.  Z.  wiss.  Zool,  1896,  Ixi,  393-419. 

221  HESSE,   R. :   II.   Die  Augen   der   Plathelminthen,   insonderheit   der 

trikladen  Turbellarien.     Z.  wiss.  Zool,  1897,  Ixii,  527-582. 

222  HESSE,  R. :  IV.  Die  Sehorgane  des  Amphioxus.    Z.  wiss.  Zool,  1898, 

Ixiii,  456-464. 

223  HESSE,  R. :  Die  Lichtempfindung  des  Amphioxus.    Anat.  Anz.,  1898, 

xiv,  556. 

224  HOGYES,  A. :  Der  Nervenmechanismus  der  assoziierten  Augenbewe- 

gungen.  I-II.  Mitt,  mathem.-naturw.  Kl  Ungar.  Akad.  Wiss. 
Budapest,  1881,  x,  1-62;  xi,  1-100.  (Ref.  Biol  Centr.,  1881-82,  i, 
216-220.) 

225  HOLMES,  S.  J. :  Phototaxis  in  the  Amphipoda.    Am.  J.  Physiol, 

1901,  v,  211-234. 

226  HOLMES,  S.  J. :  Phototaxis  in  Volvox.    Biol  Bull,  1903,  iv,  319-326. 

227  HOLMES,  S.  J. :  The  Selection  of  Random  Movements  as  a  Factor 

in  Phototaxis.     /.  Comp.  Neurol  and  Psychol,  1905,  xv,  98-112. 

228  HOLMES,  S.  J. :  The  Reactions  of  Eanatra  to  Light.    J.  Comp.  Neurol 

and  Psychol,  1905,  xv,  305-349. 

229  HOLMES,  S.  J. :  Observations  on  the  Young  of  Eanatra  quadridentata 

Stal.     Biol  Bull,  1907,  xii,  158-164. 

230  HOLMES,  S.  J. :  Phototaxis  in  Fiddler  Crabs  and  Its  Relation  to 

Theories  of  Orientation.  J.  Comfi.  Neurol  and  Psychol,  1908, 
xviii,  493-497.  ,  //' 

231'HOLMES,  S.  J. :  Description  of  a  New  Spaeies  of  Eubranchipus  from 
Wisconsin  with  Observations  on  1^  Reaction  to  Light.  Trans. 
Wis.  Acad.  Sc.,  Arts  and  Letter^  1910,  xvi,  pt.  II,  1252-1255. 

232  HOLMES,  S.  J. :  Pleasure,  Pain  and -the  Beginnings  of  Intelligence. 

/.  Comp.  Neurol.  and^Psychol,  1910fxx,  145-164. 

233  HOLMES,  S.  J. :  Evolution  of  Animal  IhfftUigence.     New  York,  1911, 

PP.296. 


LITEBATURE  187 

234  HOLMES,  S.  J. :  The  Reactions  of  Mosquitoes  to  Light  in  Different 

Periods  of  Their  History.    J.  Animal  Behav.,  1911,  i,  29-32. 

235  HOLMES,  S.  J. :  The  Beginnings  of  Intelligence.    Science,  1911,  xxxiii, 

473-480. 

236  HOLMES,  S.  J. :  The  Tropisms  and  Their  Relation  to  More  Complex 

Modes  of  Behavior.    Bull.  Wis.  Nat.  Hist.  Soc.,  1912,  x,  13-23. 

237  HOLMES,  S.  J. :  Phototaxis  in  the  Sea  Urchin,  Arbacia  punctulata. 

J.  Animal  Behav.,  1912,  ii,  126-136. 

238  HOLMES,  S.  J. :  Studies  in  Animal  Behavior.    Boston,  1916,  pp.  266. 

239  HOLMES,  S.  J.,  and  McGRAw,  K.  W. :  Some  Experiments  on  the 

Method   of   Orientation   to  Light.     J.   Animal  Behav.,   1913,   iii, 
367-373. 

240  HOLT,  E.  B.,  and  LEE,  F.  S. :  The  Theory  of  Phototactic  Response. 

Am.  J.  PhysioL,  1901,  iv,  460-481. 

241  HOWARD,  L.  0. :  Butterflies  Attracted  to  Light  at  Night.    Proc.  Ent. 

Soc.,  Washington,  1889,  iv. 

242  ILYIN,  P. :  Das  Gehorblaschen  als  Gleichgewichtsorgan  bei  den  Ptero- 

tracheidae.     Centr.  Physiol.,  1900,  xiii,  691-694. 

243  JACKSON,  H,  H.  T. :  The  Control  of  Phototactic  Reactions  in  Hyalella 

by  Chemicals.     J.  Comp.  Neurol.  and  Psychol,  1910,  xx,  259-263. 

244  JENNINGS,  H.  S. :  Studies  on  Reactions  to  Stimuli  in  Unicellular 

Organisms.     I.   Reactions  to   Chemical,    Osmotic   and   Mechanical 
Stimuli  in  the  Ciliate  Infusoria.    J.  Physiol,  1897,  xxi,  258-322. 

245  JENNINGS,  H.  S. :  II.  The  Mechanism  of  the  Motor  Reactions  of 

Paramacium.     Am.  J.  Physiol.,  1899,  ii,  311-341. 

246  JENNINGS,  H.  S. :  III.  Reactions  to  Localized  Stimuli  in  Spirostomum 

and  Stentor.  Am.  Nat.,  1899,  xxxiii,  373-389. 

247  JENNINGS,  H.  S. :  V.  On  the  Movements  and  Motor  Reflexes  of  the 

Flagellata  and  Ciliata.     Am.  J.  Physiol.,  1900,  iii,  229-260. 
24«  JENNINGS,  H.  S. :  VI.  On  the  Reactions  of  Chilomonas  to  Organic 

Acids.     Am.  J.  Physiol.,  1900,  iii,  397-403. 
249  JENNINGS,  H.  S.,  and  CROSBY,  J.  H. :  VII.  The  Manner  in  Which 

Bacteria  React  to  .Stimuli,  Especially  to  Chemical  Stimuli.     Am.  J. 

Physiol,  1901,  vi,  31-37. 
aso  JENNINGS,  H.  S.,  and  MOORE,  E.  H. :  VIII.  On  the  Reactions  of 

Infusoria  to  Carbonic  and  Other  Acids,  with  Especial  Reference 

to  the  Causes  of  the  Gatherings  Spontaneously  Formed.    Am.  J. 

Physiol,  1902,  vi,  233-250. 
251  JENNINGS,  H.  S. :  Contributions  to  the  Study  of  the  Behavior  of 

Lower    Organisms.     Carnegie   Institution    of    Washington,    Pub. 

No.  16,  1904,  pp.  256,  81  figs. 


188  TEOPISMS 

252  JENNINGS,  H.  S. :  Modifiability  in  Behavior.     II.  Factors  Determin- 

ing  Direction    and    Character   of    Movement   in    the    Earthworm. 
J.  Exp.  ZooL,  1906,  iii,  435-455. 

253  JENNINGS,  H.  S. :  Behavior  of  the  Lower  Organisms.     New  York, 

1906,  pp.  xiv  +  366. 

254  JENNINGS,  H.  S. :  The  Interpretation  of  the  Behavior  of  the  Lower 

Organisms.     Science,  1908,  xxvii,  698-710. 

255  JENNINGS,  H.  S. :  Tropisms.     Eapport  VIme  Congr.  Internal.  Psy- 

chol.  Geneve,  1909,  pp.  20. 

2550  JENSEN,  P. :  Ueber  den  Geotropismus  niederer  Organismen.     Arch. 
ges.  PhysioL,  1893,  liii,  428-480. 

256  JORDAN,  H. :  Rheotropic  Responses  of  Epinephelus  striatus  Bloch. 

Am.  J.  PhysioL,  1917,  xliii,  438-454. 

257  JORDAN,  H. :  Integumentary  Photosensitivity  in  a  Marine  Fish,  Epi- 

nephelus striatus  Bloch.     Am.  J.  PhysioL,  1917,  xliv,  259-274. 

258  JUDAY,  C. :  The  Diurnal  Movement  of  Plancton  Crustacea.     Trans. 

Wis.  Acad.  Sc.,  Arts  and  Letters,  1904,  xiv,  534-568. 

259  KAFKA,  G. :  Einf iihrung  in  die  Tierpsychologie.     I.  Die  Sinne  der 

Wirbellosen.     Leipzig,  1914,  xii+594. 

260  KANDA,  S. :  On  the  Geotropism  of  Paramcecium  and  Spirostomum. 

Biol.  Bull.,  1914,  xxvi,  1-24. 

261  KANDA,  S. :  The  Reversibility  of  the  Geotropism  of  Arenicola  Larva3 

by  Salts.     Am.  J.  PhysioL,  1914,  xxxv,  162-176. 

262  KANDA,  S. :  Geotropism  in  Animals.     Am.  J.  PsychoL,  1915,  xxvi, 

417-427. 

263  KANDA,  S. :  Studies  on  the  Geotropism  of  the  Marine  Snail,  Littorina 

littorea.    Biol.  Bull.,  1916,  xxx,  57-84. 

264  KANDA,  S. :  The  Geotropism  of  Freshwater  Snails.    Biol.  Bull.,  1916, 

xxx,  85-97. 

264o  KANDA,  S. :  Further  Studies  on  the  Geotropism  of  Paramcecium 
caudatum.     Biol.  Bull.,  1918,  xxxiv,  108-119. 

265  KELLOGG,  V.  L. :  Some  Insect  Reflexes.     Science,  1903,  xviii,  693-696. 
2«6  KELLOGG,  V.  L. :  Some  Silkworm  Moth  Reflexes.     Biol.  Bull.,  1907, 

xii,  152-154. 

267  KNIEP,   H. :   Untersuchungen   iiber  die   Chemotaxis  von  Bakterien. 

Jahrb.  wiss.  Bot.,  1906,  xliii,  215-270. 

268  KRANICHFELD,  H. :  Zum  Farbensinn  der  Bienen.    Biol.  Centr.,  1915, 

xxxv,  39-46. 

269  KRECKER,  F.   H. :   Phenomena  of  Orientation  Exhibited  by  Ephe- 

merida3.    Biol.  Bull.,  1915,  xxix,  381-388. 

270  KREIDL,  A. :  Weitere  Beitrage  zur  Physiologie  des  Ohrlabyrinthes. 

Sitzngsb.  Akad.  Wiss.  Wien,  mathem.-naturw.  KL,  1892,  ci,  469- 
480;  1893,  cii,  149-174. 


LITEBATURE  189 

271  KRIBS,  H.   G.  :   The  Reactions  of  Molosoma  to  Chemical  Stimuli. 

J.  Exp.  Zool.,  1910,  viii,  43-74. 

272  KtTHNE,  W.  :  Untersuchungen  iiber  das  Protoplasma  und  die  Kon- 

tractilitat.     Leipzig,  1864,  pp.  158. 

273  KUHNE,    W.  :    Chemische   Vorgange   in   der   Netzhaut.     Hermann  's 

Handb.  Physiol,  1879,  iii,  pt.  1,  235-342. 

274  KUSANO,  S.  :  Studies  on  the  Chemotactic  and  Other  Related  Reactions 

of  the  Swannspores  of  Myxornycetes.     J.  Coll.  Agriculture,  Imp. 
Univ.  Tokyo,  1909,  ii. 

275  LASAREFF,  P.  :  lonentheorie  der  Nerven-  und  Muskelreizung.    Arch. 

ges.  Physiol,  1910,  cxxxv,  196-204. 

276  LAUREN  s,   H.  :    The   Reactions   of   Amphibians   to   Monochromatic 

Lights  of  Equal  Intensity.     Bull.  Mus.  Comp.  Zool.,  1911,  xliii, 


277  xs,   H.  :    The  Reactions  of  Normal  and  Eyeless  Amphibian 
Larva?  to  Light.    J.  Exp.  Zool.,  1914,  xvi,  195-210. 

278  LEE,  F.  S.  :  A  Study  of  the  Sense  of  Equilibrium  in  Fishes.    I.   J. 

Physiol,  1893,  xv,  311-348. 

27»  LEE,  F.  S.  :  A  Study  of  the  Sense  of  Equilibrium  in  Fishes.  II. 
J.  Physiol,  1894-95,  xvii,  192-210. 

280  LIDFORSS,  B.  :  Ueber  den  Chemotropismus  der  Pollenschlauche.    Ber. 

bot.  Ges.,  1899,  xvii,  236-242. 

281  LIDFORSS,  B.  :  Ueber  die  Reizbewegungen  der  Marchantia-Spermato- 

zoiden.     Jahrb.  wiss.  Bot.,  1905,  xli,  65-87. 

282  LIDFORSS,  B.  :  Ueber  die  Chemotaxis  eines  Thiospirillum.    Ber.  bot. 

Ges.,  1912,  xxx,  262-274. 

283LiLLiE,  F.  R.:  Studies  of  Fertilization.  V.  The  Behavior  of  the 
Spermatozoa  of  Nereis  and  Arbacia  with  Special  Reference  to 
Egg-extractives.  J.  Exp.  Zool,  1913,  xiv,  515-574. 

284  LOEB,    J.  :    Beitrage   zur   Physiologic    des    Grosshirns.     Arch.    ges. 

Physiol,  1886,  xxxix,  265-346. 

285  LOEB,  J.  :   Die  Orientierung  der  Tiere  gegen  das  Licht  (tierischer 

Heliotropismus).    Sitzngsb.  Wurzb.  physik.-med.  Ges.,  1888. 
28  6  LOEB,  J.  :   Die  Orientierung1  der  Tiere  gegen  die  Schwerkraft  der 
Erde    (tierischer  'Geotropismus).     Sitzngsb.    Wurzb.   physik.-med. 
Ges.,  1888. 

287  LOEB,  J.  :  Der  Heliotropismus  der  Tiere  und  seine  Uebereinstimmung 

mit  dem  Heliotropismus  der  Pflanzen.    Wiirzburg,  1889,  pp.  118. 

288  LOEB,  J.  :  Weitere  Untersuchungen  liber  den  Heliotropismus  der  Tiere 

und  seine  Uebereinstimmung  mit  dem  Heliotropismus  der  Pflanzen. 
(Heliotropische  Kriimmungen  bei  Tieren).  Arch.  ges.  Physiol., 
1890,  xlvii,  391-416. 


190  TBOPISMS 

289  LOEB,  J. :  Ueber  Geotropismus  bei  Tieren.    Arch.  ges.  Physiol,  1891, 

xlix,  175-189. 

290  LOEB,  J. :  Ueber  den  Anteil  des  Hornerven  an  den  naeh  Gehirnver- 

letzung  auftretenden  Zwangsbewegungen,  Zwangslagen  und  assozi- 
ierten  Stellungsanderungen  der  Bulbi  und  Extremitaten.  Arch, 
ges.  Physiol,  1891,  1,  66-83. 

290 a  LOEB,  J.  :Untersuchungen  zur  physiologischen  Morphologie  der  Tiere. 
I.  Heteromorphose.  II.  Organbildung  und  Wachstum.  Wiirzburg, 
1891-1892. 

291  LOEB,  J. :  Ueber  kiinstliche  Umwandlung  positiv  heliotropischer  Tiere 

in  negativ  heliotropische  und  umgekehrt.  Arch.  ges.  Physiol,  1893, 
liv,  81-107. 

292  LOEB,  J. :  Zur  Theorie  der  physiologischen  Licht-  und  Schwerkraft- 

wirkungen.    Arch.  ges.  Physiol,  1897,  Ixvi,  439-466. 

293  LOEB,  J. :  Comparative  Physiology  of  the  Brain  and  Comparative 

Psychology.    New  York,  1900,  x+309. 

294  LOEB,  J. :   Studies  in  General  Physiology.     Chicago,  1905,  2  vols., 

x+782. 

295  LOEB,  J. :  The  Dynamics  of  Living  Matter.     New  York,  1906,  xi+ 

233. 

296  LOEB,  J. :  Ueber  die  Erregung  von  positivem  Heliotropisnms  durch 

Saure,  insbesondere  Kohlensaure  und  von  negativem  Helio- 
tropismus  durch  ultraviolette  Strahlen.  Arch.  ges.  Physiol,  1906, 
cxv,  564-581. 

29 7  LOEB,  J.:  Concerning  the  Theory  of  Tropisms.    J.  Exp.  Zool.,  1907, 

iv,  151-156. 

298  LOEB,  J. :  Ueber  die  Summation  heliotropischer  und  geotropischer 

Wirkungen  bei  den  auf  der  Drehscheibe  ausgelosten  kompensa- 
torischen  Kopfbewegungen.  Arch.  ges.  Physiol.,  1907,  cxvi,  368- 
374. 

299  LOEB,  J. :  Chemische  Konstitution  und  physiologische  Wirksamkeit 

von  Alkoholen  und  Sauren.     II.  Biochem.  Z.,  1909,  xxiii,  93-96. 

300  LOEB,   J. :   Die   Tropismen.     Winterstein's   Handb.   vergl   Physiol, 

1911,  iv,  451-519. 

301  LOEB,  J. :  The  Mechanistic  Conception  of  Life.     Chicago,  1912,  pp. 

232. 

302  LOEB,  J. :   On  the  Nature  of  the  Conditions  Which  Determine  or 

Prevent  the  Entrance  of  the  Spermatozoon  into  the  E'gg.  Am. 
Nat.,  1915,  xlix,  257-285. 

80s  LOEB,  J. :  The  Organism  as  a  Whole.  From  a  Physico-chemical  View- 
point. New  York,  1916,  pp.  379. 


LITEEATUEE  191 

>  j>:   The   Chemical  Basis  of  Regeneration  and  Geotropism. 
Science,  1917,  xlvi,  115-118. 

LOEB^  J. :  Influence  of  the  Leaf  upon  Root  Formation  and  Geo- 
tropic  Curvature  in  the  Stem  of  Bryophyllum  calycinum  and  the 
Possibility  of  a  Hormone  Theory  of  These  Processes.  Bot.  Gaz., 
1917,  Ixiii,  25-50. 

LoEB,  J. :  The  Chemical  Mechanism  of  Regeneration.  Ann.  Inst. 
Pasteur,  1918,  xxxii,  1-16. 

304  LOEB,  J.,  and  BUDGETT,  S.  P. :  Zur  Theorie  des  Galvanotropismus. 

IV.  Ueber  die  Ausscheidung  electropositiver  lonen  an  der  ausseren 
Anodenflache  protoplasmatischer  Gebilde  als  Ursache  der  Abwei- 
chungen  vom  Pfliiger'schen  Erregungsgesetz.  Arch.  ges.  Physiol., 
1897,  Ixv,  518-534. 

305  LOEB,  J.,  and  EWALD,  W.  F.:  Ueber  die   Giiltigkeit  des  Bunsen- 

Roscoe'schen  Gesetzes  f  iir  die  heliotropische  Erscheinung  bei  Tieren. 
Centr.  Physiol.,  1914,  xxvii,  1165-1168. 

306  LOEB,  J.,  and  GARREY,  W.  E. :  Zur  Theorie  des  Galvanotropismus. 

II.  Versuche  an  Wirbeltieren.     Arch.  ges.  Physiol.,  1896,  Ixv,  41-47. 

30 7  LOEB,  J.,  and  MAXWELL,  S.  S. :  Zur  Theorie  des  Galvanotropismus. 

Arch.  ges.  Physiol.,  1896,  Ixiii,  121-144. 

3°s  LOEB,  J.,  and  MAXWELL,  S.  S. :  Further  Proof  of  the  Identity  of 
Heliotropism  in  Animals  and  Plants.  Univ.  Col.  Pub.  Physiol., 
1910,  iii,  195-197. 

30 9  LOEB,  J.,  and  NORTHROP,  J.  H. :  Heliotropic  Animals  as  Photometers 

on  the  Basis  of  the  Validity  of  the  Bunsen-Roscoe  Law  for  Helio- 
tropic Reactions.  Proc.  Nat.  Acad.  Sc.,  1917,  iii,  539-544. 

310  LOEB,  J.,  and  WASTENEYS,  H. :  On  the  Identity  of  Heliotropism  in 

Animals  and  Plants.  Proc.  Nat.  Acad.  Sc.,  1915,  i,  44r-47;  Science, 
1915,  xli,  328-330. 

311  LOEB,  J.,  and  WASTENEYS,  H. :  The  Relative  Efficiency  of  Various 

Parts  of  the  Spectrum  for  the  Heliotropic  Reactions  of  Animals  and 
Plants.  J.  Exp.  Zool,  1915,  xix,  23-35;  1916,  xx,  217-236. 

312  LOEB,  J.,  and  WASTENEYS,  H. :  A  Re-examination  of  the  Applicability 

of  the  Bunsen-Roscoe  Law  to  the  Phenomena  of  Animal  Heliotrop- 
ism. /.  Exp.  Zool.,  1917,  xxii,  187-192. 

313  LOHNER,  L. :  Untersuchungen  iiber  den  sogenannten  Totstellreflex  der 

Arthropoden.     Z.  allg.  Physiol.,  1914,  xvi,  373-418. 

314  LUBBOCK,  J. :  On  the  Sense  of  Color  Among  Some  of  the  Lower 

Animals.  I  and  II.  J.  Linn.  Soc.  (Zool),  1881,  xvi,  121-127; 
1882,  xvii,  205-214. 

315  LUBBOCK,  J. :  On  the  Senses,  Instincts  and  Intelligence  of  Animals, 

with  Special  Reference  to  Insects.  Internat.  sc.  Series,  London, 
1899. 


192  TROPISMS 

316  LUBBOCK,  J.:  Ants,' Bees  and  Wasps.    New  York,  1904,  xiii+435. 

31 7  LUDLOFF,  K. :  Untersuchungen  iiber  den   Galvanotropismus.     Arch. 

ges.  Physiol,  1895,  lix,  525-554. 

318  LYON,  E.  P.:  The  Functions  of  the  Otoeyst.     J.  Comp.  Neurol.  and 

Psychol,    1898,    viii,    238-245. 

319  LYON,  E.  P. :  A  Contribution  to  the  Comparative  Physiology  of  Com- 

pensatory Motions.     Am.  J.  PhysioL,  1899,  iii,  86-114. 

320  LYON,  E.  P. :   Compensatory  Motions  in  Fishes.     Am.  J.  Physiol., 

1900,  iv,  77-82. 

321  LYON,  E.  P.:  On  Rheotropism.    I.  Rheotropism  in  Fishes.     Am.  J. 

Physiol.,  1904,  xii,  149-161. 

322  LYON,  E.  P. :  Rheotropism  in  Fishes.     Biol.  Bull,  1905,  viii,  238-239. 

323  LYON,  E.  P. :  On  the  Theory  of  Geotropism  in  Paramacium.    Am.  J. 

Physiol.,  1905,  xiv,  421-432. 

324  LYON,  E.  P. :  Note  on  the  Geotropism  of  Arbacia  Larva3.     Biol.  Bull., 

1906,  xii,  21-22. 

325  LYON,  E.  P. :  Note  on  the  Heliotropism  of  Palcemonetes  Larvae.    Biol. 

Bull,  1906,  xii,  23-25. 

326  LYON,  E.  P. :  On  Rheotropism.     II.  Rheotropism  of  Fish  Blind  in 

One  Eye.     Am.  J.  Physiol,  1909,  xxiv,  244-251. 
326a  LYON,  E.  P. :  Note  on  the  Geotropism  of  Paramcecium.    Biol.  Bull., 

1918,  xxxiv,  120. 
326b  McCLENDON,  J.  F. :  Protozoan  Studies.    J.  Exp.  Zool,  1909,  vi,  265- 

283. 
32?  MACCURDY,  H. :  Some  Effects  of  Sunlight  in  the  Starfish.    Science, 

1913,  xxxvi,  98-100. 
327a  McEwEN,  R.  S. :  The  Reactions  to  Light  and  to  Gravity  in  Droso- 

phila  and  its  Mutants.     J.  Exp.  Zool,  1918,  xxv,  49-106. 

328  McGiNNis,  M.  0. :  Reactions  of  Branchipus  serratus  to  Light,  Heat 

and  Gravity.     J.  Exp.  Zool,  1911,  x,  227-240. 

329  MACH,  E. :  Physikalische  Versuche  iiber  den  Gleichgewichtssinn  des 

Menschen.     Sitzngsb.  Akad.  Wiss.  Wien.,  1873,  Ixviii;  1874,  Ixix. 

330  MACH,  E. :  Grundlinien  der  Lehre  von  den  Bewegungsempfindungen. 

Leipzig,  1875,  pp.  127. 

331  MACH,  E. :  Beitrage  zur  Analyse  der  Empfindungen.     Jena,  1902. 

332  MAGNUS,  R. :  Welche  Teile  des  Zentralnervensystems  miissen  fiir  das 

Zustandekommen  der  tonischen  Hals-  und  Labyrinthreflexe  auf  die 
Korpermuskulatur  vorhanden  sein?  Arch.  ges.  Physiol,  1914,  clix, 
224-250. 

333  MAGNUS,  R.,  and  DE  KLEIJN,  A. :  Die  Abhiingigkeit  des  Tonus  der 

Extremitatenmuskeln  von  der  Kopfstellung.  Arch.  ges.  Physiol, 
1912,  cxlv,  455-548. 


LITERATURE  193 

334  MAGNUS,  R.,  and  D'E  KLEIJN,  A.:  Die  Abhangigkeit  des  Tonus  der 

Nackenmuskeln  von  der  Kopfstellung.  Arch.  ges.  Physiol,  1912, 
cxlvii,  403-416. 

335  MAGNUS,  R.,  and  DE  KLEIJN,  A.:  Die  Abhangigkeit  der  Korperstel- 

lung  vom  Kopfstande  beim  normalen  Kaninchen.  Arch.  ges. 
Physiol.,  1913,  cliv,  163-177. 

336  MAGNUS,  R.,  and  DE  KLEIJN,  A.:  Analyse  der  Folgezustande  einsei- 

tiger  Labyrinthexstirpation  mit  besonderer  Berucksichtigung  der 
Rolle  der  tonischen  Halsreflexe.  Arch,  ges.  Physiol.,  1913,  cliv, 
178-306. 

337  MAGNUS,  R.,  and  VAN  LEEUWEN,  W.  S. :  Die  akuten  und  die  dauernden 

Folgen  des  Ausfalles  der  tonischen  Hals-  und  Labyrinthreflexe. 
Arch.  ges.  Physiol.,  1914,  clix,  157-217. 

338  MAGNUS,  R.,  and  WOLF,  C.  G.  L. :  Weitere  Mitteilungen  iiber  den 

Einfluss  der  Kopfstellung  auf  den  Gliedertonus.  Arch.  ges.  Phys- 
iol., 1913,  cxlix,  447-461. 

339  MARCHAL,  P. :  Le  retour  au  nid  chez  le  Pompilus  sericeus  V.  d.  L. 

Compt.  rend.  Soc.  Biol.,  1900,  lii,  1113-1115. 

340  MASSART,  J. :  Recherches  sur  les  organismes  inferieurs.   I.  La  loi  du 

Weber  verifiee  pour  Pheliotropisme  du  champignon.  Bull.  Acad. 
Roy.  Belg.,  1888,  (3)  xvi,  590. 

341  MASSART,  J. :   Sur  Firritabilite  des  spermatozoides  dans  Poeuf  de 

la  grenouille.     Bull.  Acad.  Roy.  Belg.,  1888,  (3)  xv;  1889,  xviii. 
542  MASSART,  J. :  La  sensibilite  tactile  chez  les  organismes  inferieurs. 
J.  Soc.  Roy.  Sc.  med.  et  nat.,  Bruxelles,  1890. 

343  MASSART,  J. :  Recherches  sur  les  organismes  inferieurs.    III.  La  sensi- 

bilite a  la  gravitation.  Bull.  Acad.  Roy.  Belg.,  1891,  (3)  xxii, 
158-167. 

344  MASSART,  J. :  Essai  de  classification  des  reflexes  non-nerveux.    Ann. 

Inst.  Pasteur,  1901,  xv,  635-672. 

345  MASSART,  J. :  Versuch  einer  Einteilung  der  nichtnervosen  Reflexe. 

Biol  Centr.,  1902,  xxii,  9-23. 

346  MAST,  S.  0.:  Light  and  the  Behavior  of  Organisms.    New  York, 

1911,  pp.  410+xi. 

347  MAST,  S.  0. :  Behavior  of  Fire-flies  (Pliotinus  pyralis?)  with  Special 

Reference    to   the   Problem    of    Orientation.     /.    Animal   Behav., 

1912,  ii,  256-272. 

348  MAST,  S.  0. :  The  Relation  between  Spectral  Color  and  Stimulation 

in  the  Lower  Organisms.     J.  Exp.  Zool.,  1917,  xxii,  471-528. 
.-usa  MATULA,   J. :   Untersuchungen   iiber   die   Funktionen   des   Zentral- 
nervensystems  bei  Insekten.     Arch.  ges.   Physiol.,  1911,  cxxxviii, 
388-456. 
13 


194  .  TEOPISMS 

349  MAXWELL,  S.   S. :   Beitrage  zur  Gehirnphysiologie  der  Anneliden. 

Arch.  ges.  Physiol,  1897,  Ixvii,  263-297. 

350  MAXWELL,  S.  S. :  Experiments  on  the  Functions  of  the  Internal  Ear. 

Univ.  Cal.  Pub.  Physiol.,  1910,  iv,  1-4. 

351  MAYER,  A.  G.,  and  SOULE,  C.  G. :  Some  Reactions  of  Caterpillars 

and  Moths.    /.  Exp.  Zool.,  1906,  iii,  415-433. 

352  MENDELSSOHN,  M. :  Ueber  den  Therm otropismus  einzelliger  Organ- 

ismen.     Arch.  ges.  Physiol.,  1895,  Ix,  1-27. 

353  MENDELSSOHN,  M. :  Recherches  sur  la  thermotaxie  des  organismes 

unicellulaires.     J.  Physiol.  et  Path,  gener.,  1902,  iv,  393-409. 

354  MENDELSSOHN,  M. :  Recherches  sur  ^interference  de  la  thermotaxie 

avec  d'autres  tactismes  et  sur  le  mecanisme  du  mouvement  thermo- 
tactique.     J.  Physiol.  et  Path,  gener.,  1902,  iv,  475-488. 

355  MENDELSSOHN,  M. :  Quelques  considerations  sur  la  nature  et  le  role 

biologique  de  la  thermotaxie.     J.  Physiol.  et  Path,  gener.,  1902,  iv, 
489-496. 

356  MENKE,  H. :  Periodische  Bewegungen  und  ihr  Zusammenhang  mit 

Licht  und  Stoffweehsel.    Arch.  ges.  Physiol.,  1911,  cxl,  37-91. 

357  MEREJKOWSKY,  C.  DE:   Les  crustaces  inferieurs  distingnent-ils  les 

couleurs?     Compt.  rend.  Acad.  Sc..,  1881,  xciii,  1160-1161. 

3 58  MILLER,  F.  R. :  Galvanotropism  in  the  Crayfish.    J.  Physiol,  1907, 

xxxv,  215-229. 

359  MINKIEWICZ,  R.  i  Sur  le  chromotropisme  et  son  inversion  artificielle. 

Compt.  rend.  Acad.  Sc.,  1906,  cxliii,  785-787. 

360  MINKIEWICZ,  R. :   Le  role  des   phenomenes  chromotropiques   dans 

Fetude  des  problemes  biologiques  et  psycho-physiologiques.    Compt. 
rend.  Acad.  Sc.,  1906,  cxliii,  934-935. 

361  MINKIEWICZ,  R. :  Une  experience  sur  la  nature  du  chromotropisme 

chez  les  nemertes.     Compt.  rend.  Acad.  Sc.,  1912,  civ,  229-231. 

362  MITSUKURI,  K. :  Negative  Phototaxis  and  Other  Properties  of  Lit- 

torina    as    Factors    in    Determining    Its     Habitat.     Annotationes 
Zoologies  Japonenses,  1901,  iv,  1-19. 

s<*3  MOLISCH,  H. :  Untersuchungen  tiber  den  Hydrotropismus.    Sitzngsb. 
Akad.  Wiss.  Wien.  mathem.-naturw.  KL,  1883. 

364  MOORE,  ANNE  :   Some  Facts  Concerning  Geotropic   Gatherings  of 

ParameEcia.     Am.  J.  Physiol,  1903,  ix,  238-244. 

365  MOORE,  A.  R. :  On  the  Righting  Movements  of  the  Starfish.    Biol 

Bull,  1910,  xix,  235-239. 

366  MOORE,  A.  R. :  Concerning  Negative  Phototropism  in  Daphnia  pulex. 

J.  Exp.  Zool,  1912,  xiii,  573-575. 

367  MOORE,  A.  R. :  Negative  Phototropism  in  Diapt&mus  by  Means  of 

Strychnine.     Univ.  Cal  Pub.  Physiol,  1912,  iv,  185-186. 


LITERATURE  195 

368  MOORE,  A.  R. :  The  Negative  Phototropism  of  Diaptomus  Through 

the  Agency  of  Caffein,  Strychnine,  and  Atropin.  Science,  1913, 
xxxviii,  131-133. 

369  MOORE,  A.  R. :  The  Mechanism  of  Orientation  in  Gonium.     J.  Exp. 

Zool.,  1916,  xxi,  431-432. 

369a  MOORE,  A.  R. :  The  Action  of  Strychnine  on  Certain  Invertebrates. 
J.  Pharm.  and  Exp.  Tlierap.,  1916,  ix,  167-169. 

370  MOORE,  A.  R.,  and  KELLOGG,  F.  M.:  Note  on  the  Galvanotropic 

Response  of  the  Earthworm.    Biol.  Bull,  1916,  xxx,  131-134. 

371  MOORE,   B. :    Observations   of    Certain   Marine   Organisms    of    (a) 

Variations  in  Reaction  to  Light,  and  (b)  a  Diurnal  Periodicity 
of  Phosphorescence.  Biochem.  J.,  1909,  iv,  1-29. 

3710  MORGAN,  C.  L. :  Animal  Behavior.     London,  1900. 

37i&MoRGULis,  S.:  The  Auditory  Reactions  of  the  Dog  Studied  by  the 
Pawlow  Method.  J.  Animal  Behav.,  1914,  iv,  142-145. 

371CMORGUUS,  S.:  Pawlow's  Theory  of  the  Function  of  the  Central 
Nervous  System  and  a  Digest  of  Some  of  the  More  Recent  Con- 
tributions to  This  Subject  from  Pawlow's  Laboratory.  J".  Animal 
Behav.,  1914,  iv,  362-379. 

372  MORSE,  M.  W. :  Alleged  Rhythm  in  Phototaxis  Synchronous  with 

Ocean  Tides.     Proc.  Soc.  Exp.  Biol.  and  Med.,  1910,  vii,  145-146. 

373  MILLER,  H. :  Ueber  Heliotropismus.    Flora,  1876,  lix,  65-70,  88-95. 

37 4  MULLER-HETTLINGEN,  J. :  Ueber  galvanische  Erseheinungen  an  kei- 

menden  Sameu.    Arch.  ges.  PhysioL,  1883,  xxxi,  193-212. 

375  MURBACH,  L. :  The  Static  Function  in  Gonionemus.    Am.  J.  Physiol., 

1903,  x,  201-209. 

376  MURBACH,  L. :  Some  Light  Reactions  of  the  Medusa  Gonionemus. 

Biol.  Bull.,  1909,  xvii,  354-368. 

377  MUSSET,  CH.  :  Selenotropisme.     Compt.  rend.  Acad.  Sc.,  1890,  ex, 

201-202. 

378  NAGEL,  W.  A. :  Beobachtungen  iiber  den  Lichtsinn  augenloser  Museh- 

eln.    Biol.  Centr.,  1894,  xiv,  385-390. 

379  NAGEL,  W.  A. :  Ein  Beitrag  zur  Kenntnis  des  Lichtsinnes  augenloser 

Tiere.     Biol.  Centr.,  1894,  xiv,  810-813. 

379a  NAGEL,  W.  A. :  Experimentelle  sinnesphysiologische  Untersuchungen 
an  Coelenteraten.  Arch.  ges.  Physiol.,  1894,  Ivii,  495-552. 

380  NAGEL,  W.  A. :  Ueber  Galvanotaxis.    Arch.  ges.  Physiol.,  1895,  lix, 

603-612. 

381  NAGEL,  W.  A. :  Der  Lichtsinn  augenloser  Tiere.    Jena,  1896,  pp.  120. 

382  NAGEL,  W.  A. :  Phototaxis,  Photokinesis  und  Unterschiedsempfind- 

lichkeit.     Bot.  Ztg.,  1901,  lix,  298-299. 


. 


196  TEOPISMS 

383  NAGEL,  W.  A. :  Methoden  zur  Erforschung1  des  Licht-  und  Farben- 

sirmes.  Tigerstedt's  Handb.  physiol.  Methodik,  1909,  iii,  Abt.  2, 
Sinnesphysiologie,  ii,  1-99. 

384  NATHANSOHN,  A.,  and  PRINGSHEIM,  E. :  Ueber  die  Summation  inter- 

mittierender  Lichtreize.     Jahrb.  wiss.  Bot.,  1908,  xlv,  137-190. 

385  NihMEC,  B. :  Ueber  die  Wahrnehmung  des  Schwerkraftreizes  bei  den 

Pflanzen.    Jahrb.  wiss.  Bot.,  1901,  xxxvi,  80-178. 

38  5a  NERNST,  W.,  and  BARRATT,  J.  0.  W. :  Ueber  die  elektrische  Nerven- 
reizung  durch  Wechselstrome.     Z.  Electrochem.,  1904,  x,  664-668. 

386  NEUBERG,  C. :  Chemische  Umwandlungen  durch  Strahlenarten.    Bio- 

chem.  Z.,  1908,  xiii,  305-320;  1909,  xvii,  270-292. 

387  NUEL,  J.  P. :  La  vision.     Paris,  1904,  pp.  376. 

388  NYBERGH,  T. :  Studien  iiber  die  Einwirkung  der  Temperatur  auf  die 
tropistische  Reisbarkeit  etiolierter  J.i;ewa-Keimlinge.    Ber.  bot.  Ges., 
1912,  xxx,  542-553. 

389  OLTMANNS,  F. :  Ueber  die  photometrischen  Bewegungen  der  Pflanzen. 

Flora,  1892,  Ixxv,  183-266. 

390  OLTMANNS,    F. :    Ueber    positiven    und    negativen    Heliotropismus. 

Flora,  1897,  Ixxxiii,  1. 

391  OSTWALD,  Wo.:  Ueber  eine  neue  theoretische  Betrachtungsweise  in 

der  Planktologie,  insbesondere  iiber  die  Bedeutung  des  Begriffs  der 
"  inneren  Reibung  des  Wassers "  fiir  dieselbe.  Forsch.-ber.  biol. 
Station  Plon,  1903,  pt.  10,  1-49. 

392  OSTWALD,  Wo. :  Zur  Theorie  der  Richtungsbewegungen  schwimmen- 

der  niederer  Organismen.  Arch.  ges.  Physiol.,  1903,  xcv,  23-65; 
1906,  cxi,  452^472;  1907,  exvii,  384-408. 

393  OSTWALD,  Wo. :  Ueber  die  Lichtempfindlichkeit  tierischer  Oxydasen 

und  iiber  die  Beziehungen  dieser  Eigenschaft  zu  den  Erscheinungen 
des  tierischen  Phototropismus.  Biochem.  Z.,  1908,  x,  1-130. 

394  PAAL,  A. :  Ueber  phototropische  Reizleitungen.    Ber.  bot.  Ges.,  1914, 

xxxii,  499-502. 

395  PARKED,  G.  H. :  Photomechanical  Changes  in  the  Retinal  Pigment 

Cells  of  Palamonetes,  and  Their  Relation  to  the  Central  Nervous 
System.  Bull.  Mus.  Comp.  Zool.,  1897,  xxx,  273-300. 

3 96  pARKER>  G.  H.:  The  Photomechanical  Changes  in  the  Retinal  Pig- 

ment of  Gammarus.    Bull.  Mus.  Comp.  Zool.,  1899,  xxxv,  141-148. 

397  PARKER,  G.  H. :  The  Reactions  of  Copepods  to  Various  Stimuli  and 

the  Bearing  of  This  on  Daily  Depth-migrations.  Bull.  U.  S.  Fish 
Comm.,  1901,  103-123. 

398  PARKER,  G.  H. :  The  Phototropisin  of  the  Mourning-cloak  Butterfly, 

Vanessa  antiopa  Linn.     Mark  Anniversary  Vol.,  1903,  453-469. 


LITEEATURE       ,  197 

399  PARKER,  G.  H. :  The  Skin  and  the  Eyes  as  Receptive  Organs  in  the 

Reactions  of  Frogs  to  Light.     Am.  J.  Physiol,  1903,  x,  28-36. 

400  PARKER,  G.  H. :  The  Stimulation  of  the  Integumentary  Nerves  of 

Fishes  by  Light.     Am.  J.  Physiol.,  1905,  xiv,  413-420. 

401  PARKER,  G.  H. :  The  Reactions  of  Amphioxus  to  Light.    Proc.  Soc. 

Exp.  Biol.  and  Med.,  1906,  iii,  61-62. 

4°2  PARKER,  G.  H. :  The  Influence  of  Light  and  Heat  on  the  Movement 
of  the  Melanophore  Pigment,  Especially  in  Lizards.  J.  Exp.  Zool., 
1906,  iii,  401-414. 

403  PARKER,  G.  H. :  The  Sensory  Reactions  of  Amphioxus'.     Proc.  Am. 

Acad.  Arts  and  Sc.,  1908,  xliii,  415-455. 

404  PARKER,  G.  H. :  The  Integumentary  Nerves  of  Fishes  as  Photore- 

ceptors  and  Their  Significance  for  the  Origin  of  the  Vertebrate 
Eyes.  Am.  J.  Physiol,  1909,  xxv,  77-80. 

405  PARKER,  G.  H. :  Mast's  "Light  and  the  Behavior  of  Organisms." 

J.  Animal  Behav.,  1911,  i,  461-464. 

406  PARKER,  G.  H.,  and  ARKIN,  L. :  The  Directive  Influence  of  Light  on 

the  Earthworm  Allolobophora  fcetida  (Sav.).  Am.  J.  Physiol, 
1901,  v,  151-157. 

407  PARKER,  G.  H.,  and  BURNETT,  F.  L. :  The  Reactions  of  Planarians, 

With  and  Without  Eyes,  to  Light.  Am.  J.  Physiol,  1900,  iv,  373- 
385. 

408  PARKER,  G.  H.,  and  METCALF,  C.  R. :  The  Reactions  of  Earthworms 

to  Salts:  a  Study  in  Protoplasmic  Stimulation  as  a  Basis  of  In- 
terpreting the  Sense  of  Taste.    Am.  J.  Physiol,  1906,  xvii,  55-74. 
4°9  PARKER,  G.  H.,  and  PARSHLE.Y,  H.  M. :  The  Reactions  of  Earthworms 
to  Dry  and  to  Moist  Surfaces.     /.  Exp.  Zool,  1911,  xi,  361-363. 

410  PARKER,  G.  H.,  and  PATTEN,  B.  M.:  The  Physiological  Effect  of  In- 

termittent and  of  Continuous  Lights  of  Equal  Intensities.  Am.  J. 
Physiol,  1912,  xxxi,  22-29. 

4 11  PARMLEE,  M.:  The  Science  of  Human  Behavior.     New  York,  1913,    \/ 

xvii+443. 

41 2  PATTEN,   B.  M. :   A   Quantitative  Determination   of  the  Orienting 

Reaction  of  the  Blowfly  Larva  (Calliphora  erythrocephala  Meigen), 
J.  Exp.  Zool,  1914,  xvii,  213-280. 

413  PATTEN,  B.  M. :   An   Analysis  of   Certain   Photic   Reactions  with 

Reference  to  the  Weber-Fechner  Law.  I.  The  Reactions  of  the 
Blowfly  Larva  to  Opposed  Beams  of  Light.  Am.  J.  Physiol, 
1915,  xxxviii,  313-338. 

414  PATTEN,  B.  M. :  The  Changes  of  the  Blowfly  Larva's  Photosensitivity 

with  Age.    J.  Exp.  Zool,  1916,  xx,  585-598. 


198  TEOPISMS 

415  PATTEN,  B.  M. :  Reactions  of  the  Whip-tail  Scorpion  to  Light.     J 

Exp.  Zool.,  1917,  xxiii,  251-275. 

416  PAYNE,  F. :  The  Reactions  of  the  Blind  Fish,  Amblyopsis  spelceus,  to 

Light.    Biol  Bull,  1907,  xiii,  317-323. 
4i6apAYNE,  p>:  Forty-nine  Generations  in  the  Dark.    Biol  Bull.,  1910, 

xviii,  188-190. 
416&  PAYNE,  F. :  Drosophila  ampelophila  Loew  Bred  in  the  Dark  for 

Sixty-nine  Generations.    Biol  Bull,  1911,  xxi,  297-301. 

417  PEARL,  R. :  Studies  on  Electrotaxis.    I.  On  the  Reactions  of  Certain 

Infusoria  to  the  Electric  Current.    Am.  J.  Physiol,  1900,  iv,  96-123. 

418  PEARL,  R. :  Studies  on  the  Effects  of  Electricity  on  Organisms.     II. 

The  Reactions  of  Hydra  to  the  Constant  Current.     Am.  J.  Physiol, 
1901,  v,  301-320. 

419  PEARL,  R. :  The  Movements  and  Reactions  of  Fresh-water  Planarians : 

a  Study  in  Animal  Behavior.     Quart.  J.  Micr.  Sc.,  1902-03,  xlvi, 
509-714. 

4 20  PEARL,  R.,  and  COLE,  L.  J.:  Thei  Effect  of  Very  Intense  Light  on 

Organisms.     Third  Rep.  Mich.  Acad.  Sc.,  1901,  77-78. 

421  PEARSE,  A.  S. :  The  Reactions  of  Amphibians  to  Light.     Proc.  Am. 

Acad.  Arts  and  Sc.,  1910,  xlv,  161-208. 

422  PEREZ,  J. :  Notes  zoologiques.    De  ^attraction  exercee  par  les  odeurs 

et  les  couleurs  sur  les  insects.     Acta  Soc.  Linn.,  Bordeaux,  1894, 
vii,  245-253. 

423  PFEFFER,  W. :  Locomotorische  Richtungsbewegungen  durch  chemische 

Reize.    Ber.  bot.  Ges.,  1883,  i,  524-533. 

424  PFEFFER,  W. :  Locomotorische  Richtungsbewegungen  durch  ehemische 

Reize.     Unters.  Bot.  Inst.  Tubingen,  1884,  i,  363-482. 

425  PFEFFER,   W. :    Ueber  chemotaktische   Bewegungen  von   Bakterien, 

Flagellaten  und  Volvocineen.     Unters.  Bot.  Inst.  Tubingen,  1888, 
ii,  582-661. 

426  PHIPPS,  C.  F. :  An  Experimental  Study  of  the  Behavior  of  Amphipods 

with  Respect  to  Light  Intensity,  Direction  of  Rays,  and  Metabolism. 
Biol  Bull,  1915,  xxviii,  210-223. 

427  PLATEAU,  F. :   Recherches  sur  la  perception  de  la  lumiere  par  les 

myriopodes  aveugles.     J.  Anat.  et  Physiol,  1886,  xxii. 
42«  PLATEAU,  F. :  Nouvelles  recherches  sur  les  rapports  entre  les  insectes 

et  les  fleurs.    Mem.  Soc.  Zool  France,  1899,  xii. 
429  PLATEAU,  F. :  La  choix  des  couleurs  par  les  insectes.    Mem.  Soc.  Zool 

France,  1899,  xii,  336-370. 
4so  PLATEAU,  F. :  Experiences  sur  1'attraction  des  insectes  par  les  etoffes 

colorees  et  les  objets  brillants.     Ann.  Soc.  Ent.  Belgique,  1900,  xliv. 


LITERATURE  199 

431  PLATT,  J.  B. :  On  the  Specific  Gravity  of  Spirostomwm,  Paramcecium, 

and  the  Tadpole  in  Relation  to  the  Problem  of  Geotaxis.     Am.  Nat., 
1899,  xxxiii,  31-38. 

432  POLIMANTI,  0. :  Ueber  eine  beim  Phototropismus  des  Lasius  niger  L. 

beobachtete  Eigentiimlichkeit.    Biol.  Centr.,  1911,  xxxi,  222-224. 

433  POLIMANTI,   0. :   Sul  reotropismo  nelle  larve  dei  batraci    (Bufo  e 

Eana).    Biol.  Centr.,  1915,  xxxv,  36-39. 

434  PORODKO,  TH.  M. :  Vergleichende  Untersuchungen  iiber  die  Tropis- 

men.     I.  Das  Wesen  der  chemotropen  Erregung  bei  den  Pflanzen- 
wurzeln.     Ber.  bot.  Ges.,  1912,  xxx,  16-27. 

435  PORODKO,  TH.  M. :  II.  Thermotropismus  der  Pflanzenwurzeln.    Ber. 

bot.  Ges.,  1912,  xxx,  305-313. 

436  PORODKO,  TH.  M. :  IV.  Die  Giiltigkeit  des  Energiemengengesetzes  fiir 

den  negativen  Chemotropismus  der  Pflanzenwurzeln.    Ber.  bot.  Ges., 
1913,  xxxi,  88-94. 

437  PORODKO,  TH.  M. :  V.  Das  mikroskopische  Aussehen  der  tropistisch 

gereizten  Planzenwurzeln.    Ber.  bot.  Ges.,  1913,  xxxi,  248-256. 

438  POWERS,  E.  B. :  The  Reactions  of  Crayfishes  to  Gradients  of  Dis- 

solved Carbon  Dioxide  and  Acetic  and  Hydrochloric  Acids.     Biol. 
Bull.,  1914,  xxvii,  177-200. 

439  PRENTISS,  C.  W.:  The  Otocyst  of  Decapod  Crustacea:  Its  Structure, 

Development,  and  Functions.    Bull.  Mus.  Comp.  Zool.,  1901,  xxxvi, 
165-251. 

440  PRINGSHEIM,  E.  G. :  Die  Reizbewegungen  der  Pflanzen.    Berlin,  1912, 

viii+326. 

441  PRINGSHEIM,  E.  G. :  Das  Zustandekommen  der  taktischen  Reaktionen. 

Biol.  Centr.,  1912,  xxxii,  337-365. 

442PRZiBRAM,  K. :   Ueber  die  ungeordnete  Bewegung  niederer   Tiere. 
Arch.  ges.  Physiol.,  1913,  cliii,  401-405. 

443  PUTTER,  A. :  Studien  iiber  Thigmotaxis  bei  Protisten.    Arch.  Anat. 

u.  Physiol.,  Physiol.  Abt.,  1900,  Suppl.,  243-302. 

444  RADL,  E.:  Ueber  den  Phototropismus  einiger  Arthropoden.    Biol. 

Centr.,  1901,  xxi,  75n86. 

44 s  RADL,  E.:  Untersuchungen  iiber  die  Lichtreaktion  der  Arthropoden. 
Arch.  ges.  Physiol.,  1901,  Ixxxvii,  418-466. 

446  RADL,  E. :  Ueber  die  Lichtreaktionen  der  Arthropoden  auf  der  Dreh- 

scheibe.     Biol.  Centr.,  1902,  xxii,  728-732. 

447  RADL,  E. :  Untersuchungen  iiber  den  Phototropismus  der  Tiere.    Leip- 

zig, 1903,  viii+188. 

448  RADL,  E. :  Ueber  die  Anziehung  des  Organismus  durch  das  Licht. 

Flora,  1904,  xciii,  167-178. 

449  RADL,  E.:  Einige  Bemerkungen  und  Beobachtungen  iiber  den  Photo- 

tropismus der  Tiere.     Biol.  Centr.,  1906,  xxvi,  677-690. 


200  TROPISMS 

450  REAUMUR:  Memoires  pour  servir  a  1'histoire  des  insectes.     Paris, 

1740. 

451  REESE,  A.  M. :  Observations  on  the  Reactions  of  Cryptobranchus  and 

Necturus  to  Light  and  Heat.     Biol  Bull,  1906,  ad,  93-99. 

452  RILEY,  C.  F.  C. :  Observations  on  the  Ecology  of  Dragon-fly  Nymphs : 

Reactions  to  Light  and  Contact.  Ann.  Ent.  Soc.  Am.,  1912,  v,  273- 
292. 

453  ROMANES,  G.  J. :  Animal  Intelligence.    New  York,  1883,  pp.  520. 

454  ROMANES,  G.  J. :  Jelly-fish,  Star-fish  and  Sea-urchins.     New  York, 

1893,  x+323. 

455  ROTHERT,  W.  i  Ueber  Heliotropismus.    Beitr.  Biol  Pftanzen,  1894, 

vii,  1. 

456  ROTHERT,   W. :   Beobachtungen   und   Betraehtungen   iiber  taktische 

Reizerscheinungen.    Flora,  1901,  Ixxxviii,  371-421. 

457  Roux,  W. :  Ueber  die  Selbstordnung  (Cytotaxis)  sich  "  beriihrender  " 

Furchungszellen  des  Froscheies  durch  Zusammenfiigung,  Zellen- 
trennung  und  Zellengleiten.  Arch.  Entwcklngsmech.,  1896,  iii, 
381-468. 

458  ROYCE,  J.:  Outlines  of  Psychology.    New  York,  1903,  pp.  417. 

459  RUCHLADEW,  N. :  Untersuchungen  zur  Kritik  der  Methodik  chemotak- 

tischer  Versuche  und  zur  Biologic  der  Leukozyten.  Z.  Biol.,  1910, 
liv,  533-559. 

460  S'CHAFER,  K.  L. :  Ueber  den  Drehschwindel  bei  den  Tieren.    Z.  Psy- 

chol.  u.  Physiol.  Sinnesorg.,  1891. 

461  SCHAEFFER,  A.  A.  i  Reactions  of  Ameba  to  Light  and  the  Effect  of 

Light  on  Feeding.     Biol.  Bull,  1917,  xxxii,  45-74. 

462  SCHMID,  B. :   Ueber  den  Heliotropismus  von  Cereactis  aurantiaca. 

Biol.  Bull,  1911,  xxxi,  538-539. 

462a  SCHNEIDER,  G.  H. :  Der  tierische  Wille.    Leipzig,  1880. 
4626  SCHNEIDER,  K.  C. :  Tierpsychologisches  Praktikum  in  Dialogfonn. 

Leipzig,  1912,  pp.  719. 
462c  SCHNEIDER,   K.    C. :    Vorlesungen    iiber   Tierpsychologie.     Leipzig, 

1909. 

463  SCHOENICHEN,   W.  i    Die   Empfindlichkeit   der   Nachtschmetterlinge 

gegen  Lichtstrahlen.     Prometheus,  1904,  xvi,  29-30. 

464  SCHOUTEDEN,    H. :    Le   phototropisme    de   Daphnia   magna    Straus 

(Crust.).     Ann.  Soc.  Ent.  Belgique,  1902,  xlvi,  352-362. 

465  SHIBATA,  K. :   Studien  iiber  die  Chemotaxis  der  Zsoe'tes-Sperrnato- 

zoiden.     Jahrb.  wiss.  Bot.,  1905,  xli,  561-610. 

466  SHOHL,  A.  T. :  Reactions  of  Earthworms  to  Hydroxyl  Ions.    Am.  J. 

Physiol,  1914,  xxxiv,  384-404. 


LITERATURE  201 

467  SMITH,  A.  C. :   The  Influence  of  Temperature,  Odors,  Light,  and 

Contact  on  the  Movements  of  the  Earthworm.     Am.  J.  Physiol., 

1902,  vi,  459-486. 

468  SMITH,  G.:  The  Effect  of  Pigment-migration  on  the  Phototropism 

of  Gammarus  annulatus  S.  I.  Smith.     Am.  J.  Pliysiol.,  1905,  xiii, 
205-216. 

469  SOSNOWSKI,  J. :  Untersuchungen  iiber  die  Veranderungen  des  Geo- 

tropismus   bei   Param&cium    aurelia.     Bull.    Internal.    Acad.    Sc. 
Cracovie,  1899,  130-136. 

470  STATKEWITSCH,  P. :  Ueber  die  Wirkung  der  Induktionschlage  auf 

einige  Ciliata,     Le  Physiologists  Russe,  1903,  iii,  41-45. 

471  STATKEWITSCH,  P.:  Galvanotropismus  und  Galvanotaxis  der  Ciliata. 

Z.  allg.  Physiol.,  1904,  iv,  296-332;   1905,  v,  511-534;  1907,  vi, 
13-43. 

472  STRASBURGER,  E. :  Wirkung  des  Lichtes  und  der  Warme  auf  Schwarm- 

sporen.     Jenaische  Z.  Naturwiss.,  1878,  (N.F.)  xii,  551-625.     Also 
separate,  Jena,  pp.  75. 

473  SZYMANSKI,  J.  S. :  Ein  Versuch,  das  Verhaltnis  zwischen  modal  ver- 

schiedenen   Reizen  in  Zahlen  auszudriicken.     Arch.  ges.  Physiol., 
1911,  cxxxviii,  457-486. 

474  SZYMANSKI,  J.  S. :  Aenderung  des  Phototropismus  bei  Kiichenschaben 

durch  E'rlernung.     Arch.  ges.  Physiol.,  1912,  cxliv,  132-134. 

475  SZYMANSKI,  J.  S. :  Ein  Beitrag  zur  Frage  iiber  tropische  Fortbewe- 

gung.    Arch.  ges.  Physiol.,  1913,  cliv,  343-363. 

476  SZYMANSKI,  J.  S. :  Methodisches  zum  Erforschen  der  Instinkte.  Biol. 

Centr.,  1913,  xxxiii,  260-264. 

477  v.  TAPPEINER,  H. :  Die  photodynamische  Erscheinung  ( Sensibilisier- 

ung  durch  fluoreszierende  Stoffe).    Ergeb.  Physiol.,  1909,  viii,  698- 
741. 

478  TERRY,  0.  P. :  Galvanotropism  of  Volvox.     Am.  J.  Physiol,  1906, 

xv,  235-243. 

479  TORELLE,  E. :  The  Response  of  the  Frog  to  Light.    Am.  J.  Physiol., 

1903,  ix,  466-488. 

480  TORREY,  H.  B. :  On  the  Habits  and  Reactions  of  Sagartia  davisi.  Biol. 

Bull,  1904,  vi,  203-216. 
4«i  TORREY,  H.  B. :  The  Method  of  Trial  and  the  Tropism  Hypothesis. 

Science,  1907,  xxvi,  313-323. 
4«2  TORREY,  H.  B. :  Trials  and  Tropisms.    Science,  1913,  xxxvii,  873-876. 

483  TORREY,  H.  B. :  Tropisms  and  Instinctive  Activities.    Psychol.  Bull, 

1916,  xiii,  297-308. 

484  TORRE.Y,  H.  B.,  and  HAYS,  G.  P. :  The  Role  of  Random  Movements  in 

the  Orientation  of  Porcellio  scaber  to  Light.     J.  Animal  Behav., 
1914,  iv,  110-120. 


202  TROPISMS 

485  TOWLE,  E.  W.:  A  Study  in  the  Heliotropism  of  Cypridopsis.    Am. 

J.  PhysioL,  1900,  iii,  345-365. 

486  TURNER,   C.  H. :   An  Experimental  Investigation   of  an  Apparent 

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487  v.  UEXKULL,  J. :  Vergleichend-sinnesphysiologische  Untersuchungen. 

II.  Der   Schatten   als  Reiz   fiir   Centrostephanus   longispinus.     Z. 
Biol,  1897,  xxxiv,  319-339. 

488  v.  UEXKULL,  J. :  Die  Wirkung  von  Licht  und  Schatten  auf  die  Seeigel. 

Z.  Biol,  1900,  xl,  447-476. 

489  v.  UEXKULL,  J. :  Umwelt  und  Innenwelt  der  Tiere.    Berlin,  1909,  pp. 

261. 

490  ULEHLA,  VL.  :   Ultramikroskopische  Studien  liber  Geisselbewegung. 

Biol  Centr.,  1911,  xxxi,  645-654,  657-676,  689-705,  721-731. 

491  VAN  HERWERDEN,  M.  A. :  Ueber  die  Perception  sfahigkeit  des  Daph- 

nienauges  fiir  ultra-violette  Strahlen.     Biol   Centr.,  1914,  xxxiv, 
213-216. 

492  VERWORN,  M. :  Psycho-physiologische  Protistenstudien.    Experimen- 

telle  Untersuchungen.     Jena,  1889,  viii+219. 

49 3  VERWORN,  M. :  Die  polare  Erregung  der  Protisten  durch  den  galvan- 

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267-303. 

494  VERWORN,  M. :  Gleichgewicht  und  Otolithenorgan.     Experimentelle 

Untersuchungen.     Arch.  ges.  PhysioL,  1891,  1,  423-472. 

495  VERWORN,  M. :  Untersuchungen  iiber  die  polare  Erre>gung  der  leben- 

digen  Substanz  durch  den  konstanten  Strom.    Arch.  ges.  PhysioL, 
1896,  Ixii,  415-450. 

496  VERWORN,  M. :  Die  polare  Erregung  der  lebendigen  S'ubstanz  durch 

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497  VERWORN,  M. :  General  Physiology.     New  York,  1899. 

498  VIEWEGER,  TH.  :  Recherches  sur  la  sensibilite  des  infusoires  (alcalio- 

oxytaxisme),  les  reflexes  locomoteurs,  Faction  des  sels.    Arch.  Biol., 
1912,  xxvii,  723-799. 

499  DE  VRIES,  H. :  Ueber  einige  Ursachen  der  Richtung  bilateralsym- 

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500  DE  VRIES,  M.  S. :  Die  phototropische  Empfindlichkeit  des  Segerhafers 

bei  extremen  Temperaturen.     Ber.  bot.  Ges.,  1913,  xxxi,  233-237. 

501  WAGER,  H. :   On  the  Effect  of  Gravity  upon  the  Movements  and 

Aggregation  of  Euglena  viridis  Ehrb.,  and  Other  Microorganisms. 
Phil  Trans.  Roy.  Soc.  London,  1911,  cci,  (B),  333-390. 

502  WALLENGREN,  H. :  Zur  Kenntnis  der  Galvanotaxis.    I.  Die  anodische 

Galvanotaxis.     Z.  allg.  PhysioL,  1903,  ii,  341-384. 


LITEBATURE  203 

503  WALLENGREN,  H. :  II.  Eine  Analyse  der  Galvanotaxis  bei  Spiro- 

stomum.     Z.  allg.  Physiol.,  1903,  ii,  516-555. 

504  WALLENGREN,  H. :  III.  Die  Entwirkung  des  konstanten  Stromes  auf 

die  inneren  Protoplasmabewegungen  bei  den  Protozoen.     Z.  allg. 
Physiol.,  1904,  iii,  22-32. 

505  WALTER,  H.  E. :  The  Reactions  of  Planar ians  to  Light,    J.  Exp.  Zool., 

1907,  v,  35-162. 

SOG  WASHBURN,  M.  F. :  The  Animal  Mind.    New  York,  1909,  pp.  333. 
507  WEIGERT,  F :  Die  chemischen  Wirkungen  des  Lichts.    Stuttgart,  1911. 
sos  WHEELER,  W.  M. :  Anemotropism  and  Other  Tropisms  in  Insects. 

Arch.  Entwcklngsmech.,  1899,  viii,  373-381. 
509  WHITMAN,  C.  0.:  Animal  Behavior.     Woods  Hole  Biol.  Lectures, 

Boston,  1899,  285-338. 
5i  ODE  WILDEMAN,  E. :  Sur  le  thermotaxisme  des  Euglenes.    Bull.  Soc. 

Belg.  Micros.,  1894,  xx,  245-258. 

51 1  v.  WIESNER,  J. :  Heliotropismus  und  Strahlengang.    Per.  bot.  Ges., 

1912,  xxx,  235-245. 

512  WILLEM,  V. :  La  vision  chez  les  gastropodes  pulmones.    Compt.  rend. 

Acad.  Sc.,  1891,  cxii,  247-248. 

513  WILLEM,  V.:  Sur  les  perceptions  dermatoptiqnes.    Bull.  Sc.  France 

et  Belgique,  1891,  xxiii,  329-346. 

514  WILSON,  E.  B.:  The  Heliotropism  of  Hydra.    Am.  Nat.,  1891,  xxv, 

413-433. 

515  WODSEDALEK,  J.  E. :  Phototactic  Reactions  and  Their  Reversal  in 

the    May-fly    Nymphs    Heptageniu    interpunctata     (Say.).     Biol 
Bull,  1911,  xxi,  265-271. 

516  YERKES,  R.  M. :  Reaction  of  Entomostraca  to  Stimulation  by  Light. 

I.  Am.  J.  Physiol.,  1899,  iii,  157-182. 

517  YERKES,  R.   M. :   II.  Reactions  of  Daphnia  and  Cypris.    Am.  J. 

Physiol.,  1900,  iv,  405^22. 

618  YERKES,  R.  M.:  A  Study  of  the  Reactions  and  the  Reaction  Time 
of  the  Medusa  Gonionemus  murbachii  to  Photic  Stimuli.  Am.  J. 
Physiol.,  1903,  ix,  279-307. 

519  YERKES,  R.  M. :  Reactions  of  Daphnia  pulex  to  Light  and  Heat. 

Mark  Anniversary  Vol.,  1903,  361-377. 

520  YERKES,  R.  M. :   The  Reaction  Time  of  Gonionemus  murbachii  to 

Electric  and  Photic  Stimuli.     Biol.  Bull.,  1904,  vi,  84-95. 

521  ZAGOROWSKI,  P. :  Die  Thermotaxis  der  Paramcecien.    Z.  Biol.,  1914, 

Ixv,  1-12. 

522  ZELIONY,  G.  P. :  Observations  sur  des  chiens  auxquels  on  a  enleve  les 

hemispheres  cerebraux.     Compt.  rend.  Soc.  Biol.,  1913,  Ixxiv,  707- 
708. 


204  TROPISMS 

523  BLASIUS,  E.,  and  SCHWEIZER,  F. :  Elektrotropismus  und  verwandte 

Erscheinungen.     Arch.  ges.  Physiol,  1893,  liii,  493-543. 

524  NERNST,  W.,  and  BARRATT,  J.  0.  W. :  Ueber  die  elektrische  Nervenrei- 

zung  durch  Wechselstrome.     Z.  Electrochem.,  1904,  x,  664-668. 

525  MOORE,  A.  R. :  The  Action  of  Strychnine  on  Certain  Invertebrates. 

J.  Pharm.  and  Exp.  Therap.,  1916,  ix,  167-169. 

526  LOEB,  J. :    The  Chemical  Basis  of  Regeneration   and  Geotropism. 

Science,  1917,  xlvi,  115-118. 

52?  BREUER,  J. :  Ueber  den  Galvanotropismus  (Galvanotaxis  bei 
Fischen.  Sitzngsb.  Akad.  Wiss.  Wien,  mathem.-naturw .  Kl.,  1905, 
cxiv,  27-56. 

528  BREUER,   J.,  and   KREIDL,   A. :   Ueber  die  scheinbare   Drehung  des 

Gesichtsfeldes,    wahrend   der   Einwirkung   einer   Centrifugalkraft. 
Arch.  ges.  Physiol.,  1898,  Ixx,  494-510. 

529  HERMANN,  L.,  an/1  MATTHIAS,  F. :  Der  Galvanotropismus  der  Larven 

von  Rana  temporaries  und  der  Fische.     Arch.  ges.  Physiol.,  1894, 
Ivii,  391-405. 

530  JENSEN,  P. :  Ueber  den  Geotropismus  niederer  Organismen.     Arch. 

ges.  Physiol,  1893,  liii,  428-480. 

531  CROZIER,  W.  J. :  The  Photic  Sensitivity  of  Balanoglossus.     J.  Exp. 

Zool,  1917,  xxiv,  211-217. 

532  CLAPAREDE,  E.:  Les  tropismes  devant  la  psychologic.    J.  Psychol.  u. 

Neurol,  1908,  xiii,  150-160. 

533NAGEL,  W.  A.:  Experimentelle  sinnesphysiologiehe  Untersuchungen 
an  Coelenteraten.  Arch.  ges.  Physiol,  1894,  Ivii,  495-552. 

534  SCHNEIDER,  G.  H. :  Der  tierische  Wille.    Leipzig,  1880. 

535  SCHNEIDER,  K.  C. :  Tierpsychologisches  Praktikum  in  Dialogform. 

Leipzig,  1912,  pp.  719. 

536  SCHNEIDER,  K.  C. :  Vorlesungen  iiber  Tierpsychologie.    Leipzig,  1909. 
53?  MORGULIS,  S. :  The  Auditory  Reactions  of  the  Dog  Studied  by  the 

Pawlow  Method.     /.  Animal  Behav.,  1914,  iv,  142-145. 

538  MORGULIS,  S. :  Pawlow's  Theory  of  the  Function  of  the  Central  Ner- 
vous System  and  a  Digest  of  Some  of  the  More  Recent  Contributions 
to  This  Subject  from  Pawlow's  Laboratory.  J.  Animal  Behav. , 
1914,  iv,  362-379. 

639  CRAIG,  W. :  The  Voices  of  Pigeons  Regarded  as  a  Means  of  Social 
Control.  Am.  J.  Sociology,  1908,  xiv,  86-100. 

54<>  CRAIG,  W. :  Male  Doves  Reared  in  Isolation.  J.  Animal  Behav., 
1914,  iv,  121-133. 

541  MATULA,  J. :  Untersuchungen  iiber  die  Funktionen  des  Zentralnerven- 
systems  bei  Insekten.  Arch.  ges.  Physiol.,  1911,  cxxxviii,  388-456. 


LITERATURE  205 

542  LOEB,  J. :  Influence  of  the  Leaf  upon  Boot  Formation  and  Geotropic 

Curvature  in  the  Stem  of  Bryophyllum  calcycinum  and  the  Possi- 
bility of  a  Hormone  Theory  of  These  Processes.  Bot.  Gaz.,  1917, 
Ixiii,  25-50. 

543  LOEB,  J. :  Untersuchungen  zur  physiologischen  Morphologie  der  Tiere. 

I.  Heteromorphose.  II.  Organbildung  und  Wachstum.  Wiirzburg, 
1891-1892. 

544  LOEB,  J. :   The  Chemical  Mechanism  of  Regeneration.     Ann.  Inst. 

Pasteur,  1918,  xxxii,  1-16. 

545  CRAIG,  W. :  Appetites  and  Aversions  as  Constituents  of  Instincts. 

Biol  Bull,  1918,  xxxiv,  91-107. 

546  KANDA,  S. :  Further  Studies  on  the  Geotropism  of  Paramtecium  can- 

datum.    Biol.  Bull.,  1918,  xxxiv,  108-119. 

547  LYON,  E.  P. :  Note  on  the  Geotropism  of  Paramacium.     Biol.  Bull., 

1918,  xxxiv,  120. 

54«  MCCLENDON,  J.  F. :  Protozoan  Studies.  /.  Exp.  ZooL,  1909,  vi,  265- 
283. 

549  McEwEN,  R.  S. :  The  Reactions  to  Light  and  to  Gravity  in  Drosophila 

and  its  Mutants.     J.  Exp.  Zool.,  1918,  xxv,  49-106. 

550  pAYNE,  F.:  Forty-nine  Generations  in  the  Dark.    Biol.  Bull.,  1910, 

xviii,  188-190. 

551  PAYNE,  F. :  Drosophila  ampelophila  Loew  Bred  in  the  Dark  for  Sixty- 

nine  Generations.    Biol.  Bull,  1911,  xxi,  297-301. 

552  MORGAN,  C.  L. :  Animal  Behavior.     London,  1900. 

653  STEVENS,  N.  M. :  Regeneration  in  Antennularia.  Arch.  Entwcklngs- 
mech.,  1910,  xxx,  pt.  1,  1-7. 

554  MAXWELL,  S.  S'. :  On  the  Exciting  Cause  of  Compensatory  Move- 
ments. Am.  J.  Physiol,  1911-12,  xxix,  367-371. 


INDEX 


,  30 

Aglaophenia,  138 
Allen,  39 

Amblystoma,  41,  53,  59 
Ammophila,    170 
Amphipyra,  135 
Anelectrotonus,    32ff. 
Anemotropism,  132 
Antennularia  antennina,   119,  125 
Arbacia,  148  ff. 
Arenicola,    106,    108,    109 
Aristotelian    viewpoint     of    animal 

conduct,  17,  18 

Asymmetrical   animals,   70   ff. 
Avena  sativa,  84,  105,  106,  117 
"Avoiding  reactions,"  96 
Axenfeld,  D.,  54 

Bacterium  termo,  140,  142 
Balanus   eburneus,   75,    108 

perforatus,  116 

Bancroft,  F.  W.,  41  ff.  62,  72,  74,  98 
Barratt,  J.  0.  W.,  146,  147 
Barrows,  W.  M.,  153,  154 
Bauer,  V.,  18 
Bees,  heliotropic  reactions  of,  103  ff. 

159 

Bert,  P.,   101,   102 
Blaauw,  A.  H.,  84,  104,  106,  117 
Blasius,  E.,   32 
Blowfly,  51,  76,  109 
Bohn,  G.,  75,  82 
Brain  lesions  in  fish,  24  ff. 
in  dogs,  27  ff. 
in  sEschna,  30 
Bruchmann,  H.,  142 
Bryophyllum     calycinuni,    22,     120, 

125,  137 

Buddenbrock,  W.,  18 
Budgett,  S.  P.,  46 
Buller,  A.  H.  R.,  141,  143,  148    ff 
Bunsen-Roscoe    law,    21,    83    ff.,    99, 

100,  137 
Butler,  S.,   161 

Catelectrotonus,  32  ff. 
Centrifugal  force,  125,  126 


Chemotropism,   139    ff.,   160 

Chilomonas,   144,   145 

Chlamydomonas  pisiformis,  106,  109 

Cineraria,  48,  164 

Circus  movements,  fish,  24  ff.,  dogs, 
27  ff.,  fflschna,  30  housefly,  54, 
Ranatra,  54,  Proct acanthus,  60, 
61,  72,  Euglena,  72  ff.,  Vanessa 
Antiopa,  54 

Color  sensations,  100  ff. 

Colpidium  colpoda,   144 

Compensatory  motions,   126,   128  ff. 

"Conditioned  reflexes,"  166  ff. 

Craig,  Wv  168 

Crayfish,  38 

Cucumaria  cucumis,   125 

Cypridopsis,  116 

Danais  plexippus,   162 

Daphnia,   88,    89,   92,   96,    101,    102, 

104,  113  ff,  162,  171,  172 
Delage,  Y.,  123,  124 
Dewitz,  J.,  136,  149 
Diaptomus,   114,   115 
Dragon  fly  larva,  30 
Drosophila,    111,    116,    117,    153 

Eudendrium,  66,  73,  83,  85,  106 
Euglena,  16,  45,  62,'  70,  72  ff.,  97  ff., 

106,   109 
Ewald,  W.  F.,  85,  88,  104,  116 

"Fertilizing  149,  150 
Flourens,   P.,   27 
Forced  movements,  24  ff. 
Franz,  V.,  18 
"Fright  reactions,"  96 
v.  Frisch,  K..,  103,  104 
Fundulus,  143,  157 

Galileo,  18 

Galvanotropism,  32  ff. 
Gammarus,  113,  114,  116 
Garrey,  W.  E.,  33,  40,  41,  51  ff.,  71 

72,  132,  133 
.Gelasimus,  39,  124 
Geotropism,  119,  ff. 

207 


208 


INDEX 


Glaucoma  scintillans,   144 
Gonium,  109 
Graber,  V.,  47,  100,  104 
Groom,  T.  T.,  112 

Hammond,  J.  H.  Jr.,  68 
Harper,  E.  H.,  154 
Heliotropic  machine,  68  ff. 
Heliotropism,  47  ff. 
Hering,  E.,  127 
Hermann,  L.,  32 
Hess,  C.,  102  ff. 
Holmes,  S.  J.,  51  ff.,  73,  116 

Instincts,  156  ff. 
"Irritability,"  39 
Isoetes,  141,  142 

Jellyfish,  41,  42 

Jennings,  H.  S.,  73,  96  ff.,  119,  125, 

143  ff.,  155 
Jordon,  H.,  18 

Kellogg,  V.  L.,  158 
Knight,    125 
Kreidl,  A.,  124 
Kupelwieser,  H.,  104 

Lidforss,  B.,  142 

Lillie,  F.,  149  ff.,  15,6 

Littorwa,  75 

Lizard,  nystagmus  in,  126,  129,  130 

Lubbock,  J.,  47 

Ludloff,  K.,  43 

Lumbricus,  109 

Lummer-Brodhun   photometer,  90 

Lycopodium,   142 

Lyon,  E.  P.,  22,  125,  128,  131 

McEwen,  R.  S.,  Ill,  116  ff. 

Mach,  E.,  33 

Magendie,  27 

Magnus,  R.,  22 

Marchantia,  142 

Mast,  S.  O.,  73,  99,  108,  109,  119 

Matula,  J.,  30 

Maxwell,  S.  S.,  33  ff.,  106,  108,  126, 

135 

Mayer,  A.  G.,  162 
Mazda  lamp,  86 
Memory  images,  164  ff. 
Mendelssohn,  M.,  155 
Me'nifcre's  disease,  17,  110 
Miessner,  B.  F.,  68 


Moore,  A.  R.,  22,  112,  115,  117 

Morgulis  S.,  166 

Muscle  tension,  20  ff.;  after  brain 
lesions  in  fish,  24  ff.,  dogs,  27  ff.; 
under  influence  of,  galvanic  cur- 
rent, 32  ff.,  one  source  of  light, 
47  ff.,  two  sources  of  light,  75  ff., 
changes  in  intensity  of  light,  95  ff. 

Nereis,  135,  150 

Nernst,  W.,  46,  95 

Nernst  lamps,  76 

Neurons,  orientation  of,    38  ff. 

Northrop,  J.  H.,  75,  89,  90 

Nystagmus,  126,   129,   130 

Oltmanns,  F.,  117 

Palcemon,  124 
Palcemonetes,  33  ff.,  52,  59 
Pandorina,  106,  109 
Paramcecium,    43    ff.,    97,    125,    143, 

144,  164  ff.,  155 
Parker,  G.  H.,  54,  75 
Patten,  B.  M.,  75  ff.,  92 
Pawlow,  165  ff. 
Payne,  F.,  118 
Pfeffer,    W.,    140    ff. 
Phacus  Triqueter,   109 
Phrynosoma,    126,    129,    130 
Pliy  corny  ces,   105,   117 
Platyonichus,  124 
Polygordius,   115 
Poly  orchis  penicillata,  41,  42 
Porthesia  chrysorrhoea,  48,  116,  161 
Proctacanthus,  55  ff. 

Radl,  E.,  54,  88,  128 
Ranatra,    52    ff. 
Reflexes,  21  ff.,  166 
Retina  images,  127  ff. 
Reversal  of  helitropism,  112  ff. 
Rheotropism,  131  ff. 
Robber  fly,  55  ff.,  72 

Sachs,  101. 

Salamander    larvae,    galvanotropism 

of,  41 

Schweizer,   F.,   32 
Scyllium  canicula,  24 
Serpula,  95 
Shark,    forced    movements    in,    22, 

24  ff. 
Sherrington,  22 


INDEX 


209 


Shibata,  K,  141,  142 
Shock  movements,  97,  98 
Shrimp,  galvanotropism  in,  34  ff. 
Soule,  C.  G.,   162 
Spirillum  undula,  140 
Spirographis  spallanzani,  63,  83 
Spondylomorum,    109 
Steinach,   15.6 
Stereotropism,  134  if.,  157 
Stevens,  N.  M.,  119 
Sticklebacks,    132 

Strongylocentrotus   purpuratus,    151 
Stylonychia  mytllus,  144 
Symmetry  relations  of  animal  body, 
19  ff. 

v.  Tappeiner,  H.,  117 
Terry,  O.  P.,  44 


Thermotropism,  155 
Towle,  E.  W.,  116 
Trachelomonas  euchlora,   109 
"Trial  and  error,"  17,  73,  153,  154 
TubuJaria  mesembryanthemum,   137 

v.    Uexkiill,   J.,    18,   21,   22 

Vanessa   antiopa,   54 
Verworn,  M.,  42 
Vitalism,  18 
Volvox,  44,  45,  62,  83,  117 

Wasteneys,  H.,  86,  99,  106,  108,  143 
Wave  lengths,   heliotropic  efficiency 

of,  100  ff. 

Weber's  law,  78,  142,  143 
Wheeler,  W.  M.,  132 
Whitman,  158,  168 


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